There are two main sizes in ridable model railroads,1" (meaning 1"=1 ft.,1/12th. scale) and 1 1/2" which is 1/8th scale.
We deal with 1" scale, 4 3/4" gauge. This scale can be handled by 1 or 2 people,more practical to work with.
1" scale equipment is about 10" wide, and 13" or 14" tall. 1" scale diesels and 40 ft. freight cars will easily negotiate
a 15' radius turn.