I shall almost certainly be doing 28mm.Ah! That's easy, then. All you need to do is to look at some real 18th century pictures, and decide whether you want early coats, or later coats with turnbacks, choose some suitable miniatures, and have an idea of how big you want the units to be for the rules you're thinking of using. Given that I started with the idea that I wanted to play Charge! and use 40mm Prince August figures, those decisions were fairly easy. The consequences of choosing to use 59 figure regiments in 40mm are another story...
I think we're a little North Italian micro state, with green coats. Tempted by Wargames Factory's AWI British - their quite static sculpts seem 'right' for a unit on parade.