Most of the diesel wheels are the same size. In real life, most freight diesels had 40" wheels, some later ones were 42". Lionel, MTH, K-Line, etc. all use a wheel with an effective tread diameter a little less than 0.875". Of course, when it comes to fitting the tire, the DEPTH and WIDTH of the groove are both important. Again, IMO the diesel tires are all about the same size.
When it comes to steam locos, sadly there is no standard! Weaver and Williams brass, especially, use a very narrow tire which is no longer made. So you have to cut down an MTH tire, or make your own. MTH offers several different sizes based on driving wheel diameter. I believe there is a lookup chart on their parts site.
Personally I think rubber tires are a nuisance, and for smaller layouts, unnecessary. A little bit of wheelspin is prototypical, and in some cases permits a smoother start, but I don't advise running your train with an unfilled groove. I wish the locos came with solid wheels for the user to optionally install, so you wouldn't have the issue of periodic tire replacements. My $.02.
