Generally, the bigger canvas size for any painting, the better the detail.
Think about it this way, which is easier to draw details. A printing paper with a tiny brush or a big artist canvas. If you put too much details on a print size paper, you can get a busy face if your not careful.
mmm I don't actually know if there is a "best size", since most (if not all) artist have their own preferences.
personally, I use small canvases for doodling and sketching, around 1200x1000 with 100 pixels per inch, and when I want to start painting and detailing and increase the size to anout 7000x6500 with 300pixels per inch
but that's just me, I say, experiment, try messing around with the sizes until you find a format you like
Work at the highest resolution you can without your computer chugging. You don't want to be limited by the amount of pixels you can push around. When you're done, if you need to downsize it to fit whatever you made it for, do it then. It'll look better than if you had started off cramped.