25”. I have a Yamaha high thrust 9.9 with the 25” and it works great. It’s mounted on my bracket on a fixed mount. It tilts high enough not to drag in the water underway and the prop does not come out of the water in a bad chop or roll.
The 25” shaft is perfect, but make sure when you get the fixed mount it is the 6” or 8” tall one (measured above pod) I made the mistake of getting the 3” or 4” one and it does clear the waterline when trimmed up but when I slow down it sprays some water. So you could get away with using a long shaft with the 3-4” bracket or a XL shaft with the 6-8” bracket.