Can you do a full 360 degree turn if you move your mouse from one side of the mousepad to the other? That's kind of a starting point for sensitivity. The sweet spot seems to be 30-40 cm for a 360, from the threads I've been reading, but it depends on the game.
If your sensitivity is too low, you'll have to pick up your mouse a lot (which is fine for some players, as the video I posted above shows).