How do I change the size of my drawing in Paint Tool Sai?

I made the canvas 5000 X 5000, like a lot of tutorials suggested so i can get the utmost detail, but now that it's so big I can't post it to this site i wanted to. The tutorials usually say something along the lines of "Don't worry about the size, you can change that later" but none of them ever tell me how to change it later, so I need help Yahoo! It's probably really simple but I can't find out how to do it, when I change canvas size a ton of my drawing gets lost in the grey background. Also it's Line Art made using the pen tool, I dunno if that matters or not.