Bases are 50 by 50. I had the first 2 units on 40 by 40s (4 bases to a unit)
Then I saw some chap who based them up 4 figures on 50 by 50 bases and they looked better, less squashed.
I've done an Ancient Briton army on 40 by 40 bases and the biggest pain was squashing the figures on (each base is numbered to match up with its unit and its corresponding location in the unit)