Go with whatever is most practical, I guess. I personally ascribe to using a scale that matches travel distances of players (average movement is a 30-mile day in 5e, so I make hexes a divisor of 30 (3, 5, 6, or 10 miles) - though I also prefer to use a divisor of 40 too in case the party moves faster, so 5-mile and 10-miles hexes are good for my purposes. They can cover 6 5-mile hexes in a day, or 8 hexes if they move at 40', and 3 hexes if speed is halved, etc. Nice and round and easy to remember.