Andrew,
Gauge for loco's (larger scale anyway) is generally the distance between the rails, common sizes are 2 1/2", 3 1/2", 5" and 7 1/4"
Traction engines generally are scaled by 1" to foot so 1" scale is 1/12 full size, 1 1/2" is 1/8 full size, 2" is 1/6 full size, 3" is 1/4 full size, 4" is 1/3 full size, 6" is half!
Unfortunately the size of the model is not always a good guide to scale as with railway loco's narrow gauge prototypes are common and a 2" scale ploughing engine will be close to the same overall dimensions as a 4" scale small steam tractor!
If you want to steam any of them you will need a boiler test to get insurance to run them in public. Even if you want to use them in private it's a good idea to at least do your own pressure test before lighting up! Check any paperwork your dad may have had, it may be they have been previously tested which will make re test easier!
Paul.