People here arguing about hex sizes based on parameters that have nothing to do with the game. Should be basing it on how you want the map to look when you're done.

Draw a map that appeals to you aesthetically and put the hexes down first then figure how many days you want it to take to get to the next location. With 20-ish miles of travel a day you now know how big you want your tiles.

You want 3-4 tiles between settlements and for it to take a day to travel between? Each tile is 5~ miles. 20 tiles and 2 days travel between locations looks good to you? 2 miles a tile.

Adjust for preference. It's all arbitrary, anyway. Might as well choose the one that looks best.