I'm going to replace the quill stem on my early 90s raleight mountain bike to something that allows me to ride upright more. My bike is not accessible by me at the moment so I can't measure the stem size I currently have. When I say size I mean length.
Is there usually a default size that come stock on early 90s mountain bikes? And how is the stem measured for length? For instance, does a 130mm stem measure 13cm from the tip of the quill wedge to the top of the stem itself?

The most frequently given stem measurement is the front to back length as measured from the center of the quill to the center of the handlebar clamp. I'd guess that the most common size is 110mm but that's just a guess and there are several different common sizes.

The dimension you actually asked about is the quill height. Most stems are rather short in that dimension and have relatively little adjustment range. If you want to raise your handlebar very much you will probably need to buy something like a Nitto Technomic stem. It'll usually give you a couple of inches more handlebar height.

I'd guess that a mountain bike from the early 90's will have a 1" stem but it could very possibly be 1 1/8". If it was my bike I'd wait until I could measure it before I bought anything. Oftentimes 1 measurement is worth 1,000 guesses.

You can raise the bars on a mountan bike by using a stem with a long extension and a steep angle. This is a 135mm stem with the steepest angle I could find. Your stem is probably 1 1/8" like mine. You can tell by comparing it to a quarter. If the stem is the same diameter as a quarter, it's 1 1/8". If it's smaller than a quarter. it's 1". A long stem like a Technomic is not a good idea for a mountain bike.