When you draw at 100% all the pixels are clear and visible, without any of them deformed to fit on your computer screen.
So the best size is no bigger than your screen.
You can try everything everyone says, but the easiest to work with is really small.
If you start too small, you will need to work with each pixel, but to get more space you just add to the canvas until you find a size that is easier. Easier, works quicker, and gives you the freedom to try anything, and that makes your art and ideas better.
My canvas size varies a lot based on what I'm drawing. I try and work in standard printing sizes though, so it's easy for me to print my work if I ever want to. I've been particularly fond of 11x17inches lately, but you can work in centimeters as well. I do prefer cm/in over pixels though, for printing purposes.
The big thing you need to worry about though is dpi. If your dpi is to low your picture won't look good. I work at 300 for 95% of my stuff, but some people work even higher. Just go with what your computer can handle, since some computers slow way down if you try and go to high.
I use inches normally and use 8.5x11, so that it fits my paper for printing. I re-size some of my canvases later, the biggest has been 24x32, and I do this when I want things oversize printed. So, I mean, obviously, you just aren't going to want a size that's going to be really, really tiny.
I normally draw at about 1000 x 1000 pixels for doodles or 2000 x 2000 pixels for paintings, then re-size the canvas as necessary. (like, if I needed more vertical space, I just add to the horizontal measure and whatnot)
As for how to measure it, always measure in pixels for digital art, unless you plan on printing it out later down the line.