Austin asked: How tall can a bike be before you cannot use it on roads/sidewalks?
There is no maximum height in the statutory definition of a bicycle.
s. 316.003 – Definitions
(3) Bicycle – Every vehicle propelled solely by human power, and every motorized bicycle propelled by a combination of human power and an electric helper motor capable of propelling the vehicle at a speed of not more than 20 miles per hour on level ground upon which any person may ride, having two tandem wheels, and including any device generally recognized as a bicycle though equipped with two front or two rear wheels. The term does not include such a vehicle with a seat height of no more than 25 inches from the ground when the seat is adjusted to its highest position or a scooter or similar device.