What would you think?
What do you prefer?
Which map size should be used for 50, 100, 150, 200 and more players?
Which map size for which player amount?
It depends what type of gameplay you want to focus on. If you want a lot of PvP then a smaller map is better, to squeeze players together to encourage combat. For PvE/roleplaying they want a lot of space to build, with plenty of monuments, for which you need a larger map. 4500 is the default size which guarantees the ring road and most of the monuments. I don't think the number of players factors into it so much.