A 26' Sonic will fit under a 9' door in most cases. A lot depends on the trailer and the grade.
Play it safe, go with 10'. You are always going to want a bigger boat.
A 10' door requires a 12' ceiling height for most door applications. You do not want to try and mess around on the boat, in less than 12' period.
If you ever need more than a 10' door height, there are slim-line door applications that can roll up into the rafters, and get you higher than 10' door height, maybe 11'+.