Factory prop specs for the Tohatsu 25/30 are 9 7/8 x 11, and 9 7/8 x 12.
If your after holeshot and grip to pull a skier then your going to get a drop in top speed if you use that prop when not towing. For best towing results i reccomend getting your hands on a 4 blade prop. 4 blade props are common in things like surf use, where holeshot and cornering are the most improtant.
Biggest thing about props is that they are a compromise. A gain in one area is a loss in another area, which is when owning more than one prop is a good idea, that way you can change your prop to suit your needs.
Your current prop can be modified for a bit more top end if want to get a second prop for towing. Adding a bit more cup will give you a bit more top end. A cupped prop (in theory) will reach the same speed at a lower rpm. If you do get your existing prop modified, make sure you have it balanced afterwards, to minimise prop slip under hard acceleration.