As Greg mentioned, models are different scales but the actual size of the models (by design) are such that they mostly work well together.
Reason is that some are models of narrow-gauge equipment and others models of standard gauge equipment which are different sizes in the prototype but end up as compatible sizes in the model.
For LGB for instance, their narrow gauge models are advertised as being in scale 1:22.5 but in reality the scale varies from 1:18 to 1:24, more or less with not necessarily a consistent scale in width and height.
LGB standard gauge equipment measures closer to 1:26
Piko, almost all standard gauge items, around 1:27 except some moulds taken over
from MDC which are 1:32
USA Trains standard gauge mostly 1:29
The first Aristo-Craft and USA Trains Large Scale items were originally designed to match the size of the LGB cars so they could run with LGB equipment, but because they were models of Standard-gauge prototypes (not Narrow gauge like the original LGB), their scales working backwards ended up at 1:29 because the standard gauge prototype is larger than the narrow-gauge prototype.
Sounds a bit confusing but is quite logical if you think about it.
Bottom line: Pretty much all of the equipment by the various manufacturers looks good running together as far as the scale is concerned.
Small size variations don't matter because if you look at the prototype, you will also see boxcars with different height and obviously different lengths on the same train.
Possible incompatibility to watch out for are the US style knuckle couplers. Some manufacture's work well together, others don't - tendency is to replace knuckle couplers from the same manufacturer on all rolling stock.
On the other hand, the LGB Hook and Loop couplers of all manufacturers work well together as far as I know.