If the figures need more than 20mm because they are 'stepping forwards' like archers or spearmen then pill shaped bases will work. You reckon you have OCD! I cast my bases in moulds based on 3D printed masters and have I think 7 different sizes of pill shaped bases to accommodate long thin figures like lizardmen and reptiliads etc as well as the aforementioned archers etc.

If the figures are doing the 'splits' (note 1) you have to get bases that are wider, but there are tricks you can use in the movement tray.
1. 'Counts as' If your base is supposed to be 5x20mm frontage (100mm) have 4x25mm and count it as 5 figures.
2. If you have to have the correct number of figures, stagger the ranks so that as for the above example you use 5x25mm like so:
.
x x x.
x x(The ASCII art above is meant to show one line of figures, with every second one pushed back a half base depth and the whole lot compressed laterally to fit in the 100mm frontage.)
So two ranks of 5 would look like this:
.
x x x.
x x.
x x x.
x xHope that helps.
Note 1: Why figure sculptors, why so many Elvis impersonators? It's not like the splits is a common fighting stance ... Not to mention that your toes are typically still pointing forwards, whereas a significant number of models have East and West pointing pinkies ... (ouch!)
Cheers!
Joe Thomlinson