If you plan on using the circular black plate - that normally holds the clutch mechanism in place - to secure the motor once it's in the mount, then 25mm is about right. The neck of the motor is about that size when you factor in the screw holes. I never use thicker than 20mm for chassis/robot bits so I usually have to add a ~4mm spacer on to it so that when I put the screws in, the metal plate gets tightened against the chassis, if that makes sense.