I know some of you are probably thinking.. "But wait? Aren't sci fi and fantasy inherently unrealistic?"
Well... yes and no. Fantasy is inherently unrealistic, but sci-fi can vary (as discussed in my topic on sci-fi hardness).
The purpose of this topic is to discuss how to make spaceships, mecha, and other stuff applicable in a realistic setting that is somewhat believable and plausable, if not necessarily feasible by current reckoning. When coming up with a good sci-fi universe for stories, you really need to start from the ground up. When multiple writers are involved, people may have very different views of what they find "cool" when it comes to sci-fi, and without a universal standard, it becomes an inconsistent space opera, like Star Wars... fun, but not realistic at all.
So how to design ships, mecha, and everything else while keeping it realistic? Well, it's largely up to the GM to set the standards. Realistic sci-fi universes require a lot more GM regulation than unrealistic ones. Since I use a more realistic setting than most, I figure I'll share some ideas.
A lot of people like to imagine spaceships as being huge, massive pieces of metal slowly drifting through space, except when they go FTL. This is far from accurate, in a realistic setting. If you look at modern ships... they're actually quite small and compact in comparison. And that's comparing spaceships to ocean-navies... a very bad comparison. About half of a naval warship is submerged under the ocean, and they have to worry about drag and current much more than a spaceship does. They're also quite slow, whereas a spaceship must be FAST... because space is BIG.
A more accurate comparison would be aircraft. While aircraft still have to worry about atmospheric drag, they're built for speed, compact in size, and highly maneuverable. Increase their altitude by a few thousand miles, and you have spacecraft. Such as the space shuttle, or Saturn rocket. Currently, no one has build anything much larger than the Saturn rocket designed for high-speed maneuverability in space... but you can imagine it would have a more compact design, with a lot of thrust from the engine. Since speed is determined by thrust divided by mass, you actually want as little mass as possible in order to achieve high speeds. The best ways to reduce mass are either to reduce volume or reduce density. Reducing density involves adding in a ton of empty space, which is highly inefficient. Reducing volume makes the ship much smaller than what you would see in Star Wars or WH40k.
Another thing writers often overlook is the fact that reality is three-dimensional. They measure ships by length. Length is NOT a good way to measure a ship.. as it's only one dimension. Mass is a function of volume, which is a function of three separate dimensions. To double a ship's length is to increase their volume and mass by 8. To multiple a ship's length by 10 is to increase the volume a thousand times over. That Executor from Empire Strikes Back? Yeah, it's a thousand times the mass of a star destroyer. In a realistic setting, it would take the combined firepower of a thousand star destroyers to match it. I didn't see a thousand star destroyers in that fleet... couple dozen or so, maybe even a hundred, but not a thousand.
A lot of people like to measure the "length" of ships in kilometers. This is not a realistic measurement for many different reasons. The physics reason is that according to square-cube mechanics, the thrust of a ship that massive should disintegrate it. The economics answer is that it's just too bloody expensive. Take for example the Imperial-class Star Destroyer. It's something like 1.6km in length. Now, compare it to a space fighter (which is very realistic in space) which costs the equivalent of, say, $50 million and is 16 meters in length (also pretty realistic). Using those same dimensions, the star destroyer is 100 times the length, or 1 million times the volume (100 cubed). Even if you halve the cost of the fighter, you're still looking at a single ship that costs $25 trillion.. nearly half the GDP of Earth. To make anything several kilometers in length? Do the math, remembering to cube the dimensions, and you'll find out that it's just not realistic.
Keeping in mind how MUCH space is out there, it's likely that battles between space fleets will involve hundreds if not thousands of warships. Because a space empire needs to be able to defend a vast amount of territory. The number of ships can be reduced for smaller scale space empires.. such as between single-system countries, but the actual size of most warships is unlikely to be affected much... larger empires will simply have more of them, as they have more space to cover. Ships the size of modern navy ships are much more realistic for space combat. Get too much bigger than a Nimitz-class supercarrier, and you're stretching the limits of what is realistically feasible.
The very concept of Mecha is that they are based on "rule of cool" mechanics. Because what's more cool than a giant humanoid robot blowing up another giant humanoid robot with a giant space gun? Two giant humanoid robots blowing up two other giant humanoid robots, that's what!
However, mecha by their design are inherently unrealistic. This does not mean that they can't exist in a setting... they just wouldn't be the super-weapons that they are often portrayed as in anime. Pieces of military equipment is all they'd really be... real-robot genre rather than super-robot genre, basically.
But even real-robot genre can be unrealistic. There are several factors to overcome when making mecha to begin with.
First off, standing on two legs is inherently unstable. Ever try to make a two-legged table, or a two-legged chair? It requires amazing balance just to be able to stand upright... and it's also very easy to knock a two-legged object down. Bigger feet do add to stability, but more feet adds much more stability. However, since most people thing of humanoid mecha when they think of mecha, I'll focus primarily on two-legged mecha.
A stable robot might use three or more legs for increased stability. Or if you want to make them cool, just give them large enough legs and feet to support their massive size. Square-cube mechanics make it difficult for huge robots to stand on two feet without collapsing under their own weight, so the legs would likely be proportionally larger than human legs.
Another thing to consider is that standing upright makes you a larger target. One of the first thing soldiers do in combat is drop to a prone or kneeling position, making themselves a smaller target, and take cover behind objects. Upright mecha would have much greater difficulty in doing so, as large size makes them inherently unwieldy. Not to say it can't be done, but it takes longer.
Also, the humanoid form has a great deal of surface area... apparently evolved due to the fact that the tropical environment of Africa where humans are said to have originated from is super hot. Also the reason we have to wear jackets in the cold... too much heat escapes from our high surface skin. The problem with having a high surface area is that it is difficult to put armor on a humanoid form. Since armors protection is measured by its thickness, in order to provide sufficiently thick armor on a humanoid from, it has to be a whole lot more massive than, say, the same thickness on a sphere. So mecha are likely to be fairly lightly armored... not heavily armored. Or else they'd collapse under their own weight.
There are many other examples to consider when designing realistic mecha, but suffice to say that they're very different than what most people think of. They can be done, but a lot of realistic-setting countries probably wouldn't bother... tanks are more efficient, if less maneuverable.
Of course, you might also consider removing the humanoid form from mecha entirely and just use multi-legged mecha. Spiders and Scorpions are rather efficient designs, many legged and easily armored. Send in the spider mechs! Take on the roboscorpions!