I'm planning on purchasing a mylar curtain. What is a good size to have it made so that it will fill the typical stage?

So far I have not been able to determine what a typical stage is as sizes vary a lot. If you're gonna have one made, maybe you should go with a modular curtain (like 3 or 6 feet wide curtain pieces) so you can assemble the proper width on site.

That's what I was thinking... I'm more concerned about the best height.

That would depend more on what you are going to perform in front of it. If its a levi, how high will it go? Two stages I have used a lot vary a lot in widht and height as well, one being about 15 feet high (height being the portal height) while the other is about 25 feet high and I made a separate curtain for each (sort of a mylar curtain on a budget).