Moka pots are great for many reasons. One of those reasons is that you can get good coffee out of almost any reasonable grind size. My recommendation would be to play around with it, taking notes on grind variables and resulting taste profiles.

If your grinder is a blade grinder, I would recommend to grind to just as the grounds start to stick to the bottom. For me, that's about 20± seconds for the 4 mark. I only usually use a blade grinder at work, and only sometimes, as I prefer a burr grinder. Don't grind finer than espresso, or you risk a pipe bomb type of explosion. Moka pots can be subjected to massive amounts of pressure during the brewing process, and too fine of a grind can clog the release, essentially creating a bomb (one of the scariest days of my life).