I fear that there isn't really a "correct" answer to this question: it's almost impossible to tell how big a waffle iron is (in terms of how much batter it holds) without just experimenting to see how much batter it holds. The amount you need will depend on the size of your waffle iron, whether it's round or rectangular, and how deep it is.
Similar to what other people have said, I figured it out by trial and error. I use an almost-full ladle (scraping the bottom of the ladle over the lip of my mixing bowl to prevent drips) and pour the batter to the middle of the pan. That's the perfect amount for my waffle iron.
I actually don't recommend spraying the outside of your waffle iron with non-stick spray/oil. In my experience, it makes things messier in the long term. The oil gets kind of cooked on because it's constantly being heated.
To make cleaning easier, just clean it as soon as you're done. If your waffle iron is hot enough, any drips on the outer part may have cooked to a crisp, meaning you can just scrape them off with a spatula or knife and be more or less done. The other thing I do that works on mine is wipe it down with a sponge or damp paper towel while it's still warm (just be careful not to burn your fingers, and don't use a plastic scrubby sponge that might melt on contact). If there's a lot of grease on the outside of the pan, a drop of dishwashing soap on the damp paper towel or sponge should help.