The size of sprite really depends on the platform your game is for, and type of game.
You can get away with having smaller sprites on PC than console because you are closer to the screen and can clearly see more of the smaller details, making the smaller sprites easier to see. UI elements are another thing that is generally smaller on PC, as the small text is almost unreadable on console but perfectly fine on PC.
As for game type, fighting games tend to favour larger sprites because the backgrounds are usually not important, but seeing the minute details in the character is.
For a platform game, knowing what is coming and having time to react is more beneficial, so the characters are smaller and the environment takes priority.
Handheld games tend to have a smaller screen and less resolution, so the characters are slightly larger than they would be for a given game type, which makes it easier to see the characters moving around.
A method I use is to take a screenshot of a game of the same type and platform that I am making, measure things like hud elements and character sizes, and use that as a base measurement for my game. You can take some creative liberties of course, but usually those sizes feel pretty good.