Here is an example in the photo below: These are "models" of the same locomotive, basically:
Left, Lionel, traditional size (ATSF Annissary set from 2000). R
Right: MTH, scale (PS1 era Premier)
Both the A and the B units in each set are nearly identical in length it is perspective that makes the B units look longer.
The Lionel A units are 12.5 inches long (sans couplers)- the MTH are 12.625
the Lionel A units are 3 3/8 high versus than the MTH (3 17/16)
In width - I can meausre not difference.
If you assume the MTH is a 1:48 scale model, then the Lionel is 1:48.5 in length, 1:52 in height and 1:48 in width.
The Lionel F3 is an example of a "traditional" loco that is so close to scale that I have no problem running it on my "scale" layout. Many traditional steamers are much more compressed, maybe 1:53, 1:60, and 1:50 in length, height, width. Generally, my expedrience is that height is always compressed most, and length much more than width