Requirements vary according to where you're submitting to. Check publishers' websites to see their exact style requirements. Where they aren't specified, Times Roman or Courier New 12pt is the accepted norm - the latter more so because it's a mono spaced font (although I'll admit it looks awful from a typographic point of view). That is, an I uses the same amount of space as an M and thus an editor can more easily gauge how many words/pages long your printed book is.
Whatever font you use, optimum length of chapter is the same as the length of a piece of string. It will be as long as it needs to be in order to achieve its aim and has nothing to do with what font you use.
Whilst you're doing the actual writing, however, you are free to use whatever font you like - I often choose a fancy one that suits the mood of the book, then change it as necessary before I submit it anywhere, or when I get tired of it. That's the joy of a word processor.