Lets say I am creating an icon in illustrator, which should be saved as an svg. If I create a large nice vector icon, lets say 800x800 pixels, I can always scale it as I want to, and its "easier" to work with. Problem is, when this icon is saved, the file size is big. If I instead make a small icon in illustrator, on a document size which has the proportions the icon will be used as, lets say 20x20, the file size is much smaller.
So what is good practice when working with icon sizes? Thanks!
You are mistaken. A vector graphic that is 800x800 should have the same file size (more or less) as a vector graphic that is 20x20. That is because the shapes that make up the file are defined "mathematically" and can be rendered at any size.
If your files have significantly different sizes, you have probably not created a purely vector file. It probably has bitmap images in it. Bitmap image file sizes are greater if the image size is larger.
However, in answer to your primary question, it doesn't really matter what the page size is when creating vector icons. They could be either of the two sizes you mentioned. Or any other size. Whatever dimensions you choose, they should be rendered nicely at any final size you need.