You don't have to fill the entire canvas. It's good to have some extra space so that you can make your image as large or small as it wants to be, and you can always crop out just the part you need later.

As for the general size to use, that really depends on what you want to draw, doesn't it? There's no one particular size or subject that's good for beginners. Pixel art is just like any other medium - you can draw whatever you want. Perhaps start with things you're already comfortable with drawing, to get the hang of the medium's quirks?