Traditionally speaking the handle of a daito katana (up to 3 shaku) is around 35% of the blade's total length. For instance: the handle of a 2.3 shaku sword (about 70 cm) can have a tsuka of around 24.5 cm. A sword with a nagasa of 2.6 shaku(around 79 cm) can have a handle of 27.65 cm.
Please understand that this is only a guideline and does not count the fact that modern practitioners can have much larger hands than the classic samurai. Also some schools have a preference for longer sword handles.
However in the most iaido and kenjutsu schools you can use the following rule:
The handle should not be longer than two fists and 2-3 fingers.
A shorter or longer tsuka will change the balance and leverage of the sword and with this; reducing the efficiency of the Japanese sword techniques. Also one should be able to stab with the sword (ha upwards) by pushing on the butt-end of the handle (kashira), without having to change the grip of the right hand.