Work out the garage dimensions needed for one or two cars.
A garage that only just fits the car is the classic false economy, so the width is built up from vehicle widths plus a clearance either side and between cars: 3 ft per gap lets a door open fully and a person walk through. Two 6.5 ft cars with 3 ft gaps therefore need 25 ft of internal width, which is why the common 20 ft double garage feels cramped. Depth is the vehicle length plus about 3 ft in front for walking round the bonnet and 1.5 to 2.5 ft at the back for storage or a bench. Door width is separate: 9 ft for a single, 16 ft for a double, and 18 ft where you want two cars through one opening without clipping mirrors.
Garage Size
Width = cars x vehicle width + (cars + 1) x side clearance; depth = vehicle length + 3 ft front walkway + 1.5-2.5 ft rear storage
It parks two cars and little else — about 2 ft of clearance per gap, so doors open partway and there is no room to walk between. 24 by 24 is the size most people wish they had built.
A full-size crew cab is 19-20 ft, so allow 24 ft of internal depth. A 20 ft garage will not close its door behind one.