To cover all possible boats that you can tow without a permit than a garage of about 14 7 feet high would cover all the bases.
Boat garage door height.
As a rule of thumb you should have at least a foot of clearance for safe movement and storage when you back the trailer and boat back in to the garage.
As with the garage length this will depend on the height of your pontoon and trailer considering the bimini too.
Garage door height is a key limiting factor for most boats excluding bass boats.
Most ocean boats say bay boats or pontoons have real issues with garage doors since they are typically tall.
But do not have to put the tower down and have plenty of room to walk around and even store stuff behind the boat.
If your boat is above a certain height you need a permit to tow it.
Bass boats are generally very low profile and only power poles if applicable fishing seats and huge outboard motors ever come close to not clearly.
Get a bigger garage.