You can actually upload a 20GB file to YouTube. Ultimately, once YouTube gets your file they will recompress it again. So playability isn’t as much of an issue since they generate different version of it automagically on your upload (360p, 480p, 720p if HD is uploaded and 1080p if so uploaded).
How long is your video? The biggest impact on your file size is the data rate of your video and the data rate of your audio. The higher they are the bigger the file. How that impacts quality depends on the overall frame size you’re dictating. So if you have a frame size of 1280×720 with a data rate of 200kbps total (both audio and video together) than you’ll have a small file, but poor quality. Conversely, if you have the same 720p frame size, but a total data rate of 5,000kbps then you’ll have great quality and a big file size.
I like Vimeo’s compression guide. I think it’s a good starting point for most people:
https://vimeo.com/help/compression
Ryan Holmes
http://www.ryanholmes.me
vimeo.com/ryanholmes