As I understand it:
3/4"' scale (3/4" to the foot) locomotives typically run on 3 1/2" gauge track. This is about the most universal "standard" of any of them.
1" scale locomotives are typically built for either 4 3/4" gauge or 5" gauge track. 4 3/4" gauge is older and I think limited mostly to the east coast.
1 1/2" scale locomotives are typically built for 7 1/4" gauge or 7 1/2" gauge track. Again, one is more common the east coast and the other on the west coast.
There are also smaller gauges -- 2 1/2" gauge track, etc.
And, as others have said, these are not absolute relationships, if you start considering 2' gauge prototypes and such.
Probably the first thing to do is decide what gauge track you're going to run on -- what your local railroad club has, for instance -- and then find locomotive plans to suit that gauge. Plans can be scaled, but it is not always straightforward. When you scale the boiler, it will probably need to be redesigned for proper operation and/or safety at the different size.