Another consideration if you are designing and building from scratch is get as large a room as possible. Then add a few feet! Our trains require a lot of room to fully enjoy. We used to joke that we really need a small aircraft hanger for O gauge. Actually, there is an airport community north of you in New Port Richie that is 40 years old and another SE of Ocala called Love's Landing - might be something to consider. Lol
The typical double garage is really too small for 3 rail O unless you are happy with tight curves and limited trackage. Even 10 extra feet will make a huge difference over time.