If you are referring to the photographs used in the catalogue, they are 60mm x 30mm.

I am working on an AWI project that is focused on British Grenadier, but I find the basing works for a lot of different rulesets.  My method is to replicate the frontage of the bases used for 28mm figures, but using 6mm figs for the mass effect and less unit depth.  Its an expesive way to go, because you only get like 1 unit from a Baccus pack, but I am very happy with the result (at least for my purposes).  

I am doing 40x15mm for the formed infantry bases.  Each represents 4x 28mm figures as would be mounted on a 40x40mm base.  I have some 20x15mm bases which represent 2x 28mm figures that can be used to reflect casualties or odd-sized units.  4-5 bases per regiment/brigade.  I am generally not painting facing colors on my redcoats, so that the diferent units can be easily represented simply by swapping out command bases with different colors. 

Skirmishers are mounted on 20x15 or 25x20 bases with 2-3 6mm figures on each.  Each represents a single 28mm skirmisher figure.

I am mounting 3 6mm cavalry figures on 25x20mm bases, each of whch represents 1x 28mm cavalry figure.   

For artillery, I am doing 60mmx30mm bases with 2 guns and crews on each.  

It makes sense to me at least!  

If I could figure out how to post photos on this site, I would.