It should be as big as you need and want it to be. However, if you want to make atleast 1mil per floor I suggest making a 9 or 16 claim base.
The best thing for smaller factions without to much Power should do a 32x32 - 42x42 and have it super tall, so its less claims.
Make it so you would get at least 3 invs of wart a floor, and if you make it tall enough you will make a lot.
As stated above, if you're low on power or resources, just make it 32 by 32 and really tall. When you get richer, make the farm much wider and longer, but also try and keep it as tall as you can.
If you don't like hacking, you may want to get a farming client anyway. Enjoy a server that hacking is permitted while you can