I hope that you'll look at the WSS materials at warflag.com
There were two big battle projects, Blenheim 2004 and Ramilles 2006, that are documented there. Blenheim 2004 was played on a 6 ft x 32 ft table with 32 players. Ramilles 2006 was played on a 6 ft x 28 ft table with 20 players. In order to do that a fast play easy set of rules was developed to allow for such big battles. I think the rules can be downloaded from that website. Those projects used 25/28mm figures. The figures were Foundry, Irregular, Old Glory, Imperialist Enterprises, Front Rank, and a few others. Not sure if we had any Dixons painted.
If you don't go with 25/28mm, I'd suggest 10mm. That's not because of my personal experience, but because of the observation of a friend of mine that one is obligated to paint the detail on 15/18mm figures, but the detail on those figures become more difficult to see without a magnifying glass as one ages. The time input to speed paint 10mm is much less than the time required to paint 15/18mm or 25/28mm.