Hi Swampy,
I always go for a 'standard' size, depending on the nationality and period (32 figs on four company bases in the case of Prussians) and just alter the unit roster according to the scenario - a slight discrepancy in frontage doesn't matter a bugger really. My 32s regularly 'stand in' for 40s or 24s (or occasionally hyper-accurate 27s, 31s, etc, depending on the scenario).
You can always use the argument that as units became smaller due to attrition, the third rank would be used to fill out the first and second ranks to maintain the frontage.
Cheers,
Mark