what size shaft length?

[tsubaki]

You are looking to get a bracket on a full transom. You can choose whatever motor shaft length you want.
Wowever, a 25" will allow you a lot of play room as to not drowning the powerhead during normal operations.
When you get the bracket and motor, ask these guys here that have done a bracket installation and they will give you some measurements to install the stuff at.
25"!!