Its been mentioned to me that I should describe what we mean by strength:

Its not really about physical or even Force Strength. These things help, but if there was any one thing you are judged on it is your will and determination to continue. For all other things, the teachers don't really care. The Sith Academy accepts pretty much anyone who wants to join (though at the present point in history its a little race dependant) because the way in which 'teaching' is carried out ensures that anyone not determined enough to be a Sith ends up being dead. As long as you make it through your lessons alive, you are 'strong enough.' Wether you managed this by being a brute who is unchallengable in lightsaber combat, a Force adept of great strength, a manipulator of others or just really really lucky, it doesn't matter to the teachers. What matters is being alive at the end of the day.

As with every rule, there are exceptions, but thats generally how it works.