To get back to the question, there really isn't a standard for glow plug sizes or heat. Someone's "hot" plug may be someone else's "cold" plug.

It's tough to figure out which plug to use when you're starting out. You may ask, "Well, why isn't there a website with all this data listed?" That would require someone, one person, to go out and buy every glow plug available, find some way to test and compare them scientifically, compile the data, build a website, and advertise it. That's a LOT of work for someone to do on a VOLUNTEER basis, paying for all those plugs, and the website, out of his own pocket. It would have to be one person, or a carfully organized group. We couldn't just send in reports, it wouldn't be accurate... "Oh I think this plug's hot." Well, what's hot? Even the plug manufacturers don't know. Besides, a community project where there is no clear goal and no reward for the participants is difficult to organize, and even more difficult to sustain. You'll get lots of "I'll help" but when it comes time to do the work or send in the data, forget it...

Anyway, once you get into the hobby, you'll develop a sense for what glow plugs you like. It's a very subjective thing. I've tried probably a dozen different models of glow plugs, and my personal favorite is the OS #8. It's expensive, but reliable.