The general rule of thumb is that the tsuka should be roughly 1/3 the length of the blade, but there are exceptions. So 30" blade should have roughly a 10" tsuka. There are at least a small number of styles which prefer longer tsuka, and shorter is not unheard of either. This is an "Ask your sensei" question.
_________________________
Iaido -
Breaking down bad habits,
and building new ones.