SCALE

True scale for 10 1/4" Gauge is approximately 2 3/16" to the foot - however, perhaps the most common scales used are those shown below:

As can be seen, the increase of scale to (say) 2 1/2" to the foot leads to a significant up-sizing, which can be most useful when an engine is intended to haul heavy trains, or needs a full size driver or a diesel engine fitting in it....

Ultimately, individuals must make up their own minds as to what scale suits them best - there are fine examples of loco's running built to all of these sizes....