Geof says:

Gary,

By size I assume you actually mean style. Most HO rolling stock & locos comes equipped with the so-called horn hook coupler or X2F (I think it was once called). Anyway, the X2F is cheap and doesn’t look at all realistic but it works fairly well.

Another type that sometimes comes installed on some brands is called the knuckle coupler. Though a bit too large to actually be in scale, it looks and operates very much like an actual coupler. Kadee is the name of the company that makes these couplers although I believe there a handful of others out there that make couplers that are compatible with Kadees. Kadees can be installed on most (if not all HO equipment) but the process can run from the extremely easy to the extremely difficult. Generally speaking, installation on locos is a little more difficult than on rolling stock.

Hope this gives you a starting point. There are other posts on this topic so you may want to use this blog’s search engine to find them.

Geof