[@Silent Observer] What I like to do is if a gif is around 250 x 250 and the url is Tumblr (or rebloggy usually) is that I visit the site the image is on and find that they're usually part of a set.

[img]http://25.media.tumblr.com/ca4c465a7ce49b8bcbad2f9536faef9b/tumblr_mk9chv0YXt1rkhucho4_r1_250.gif[/img]

This one of Sebastian, for instance, is from [url=http://rebloggy.com/post/my-gifs-labyrinth-sebastian-stan/46644572628]here[/url]. Boom that's a set of gifs that are sized exactly.

Usually, if it's a set they're all made to be the same size. If they aren't a part of a set, but you'd still like to use the image, it's only a matter of narrowing your google down to the exact size of that gif. It'll usually tell you the size if you prompt it to or if you click on the image to enlarge in within the search.

If you're still having trouble, any one of us would be more than happy to help look for gifs for you! :o I love gif hunting because I'm addicted to gifs, as you can tell. ;3;