Newline designs Napoleonic figures are stocky similar in style to Strelets old sculpts but a bit shorter 20-22mm (measuring from sole of foot to top of head usually half way up shako).
Hagen Miniatures I have Belgian, Dutch, Bavarian, Scots Grey Cavalry and French and Dutch Infantry all are very nice but a bit on the large side 25mm-25.5mm there are exceptions their Spanish Artillery I have are 23mm and mix very well.
Franznap I have Austrian Hussars, Austrian and Bavarian Artillery all are 23mm and mix really well.
All Metal figures are quite expensive when compared to plastic figures with Newlines being cheaper than most metal figures but all of the above will have offers or consider doing deals if you email them.
As nice as the Metal guns are I decided to save money and use plastic guns for my Artillery and you could always convert your plastic figures to make a desired unit.
The 20mm are smaller, but stick a bit of card to the base and they sit well enough (tried and tested with the Colonials).
Of my several thousand painted figures, I guess about 40%+ are metal, from several different manufacturers. I think, unlike some of our members sticking a new toe in the 'mix 'n' match waters, I can claim some expertise.
I agree with your post, Alan, and would suggest it applies also to Naps, Ancients and WW2 as far as Newline is concerned but also as a general rule of thumb for most small disparities in size between ranges.
My advice is that girth matters more than height when matching figures. You can, if necessary, conceal differences in height but if some figures are notably heftier or slimmer, this is far more obvious & hard to hide.
I should also add a lot lies in individual taste: what I can live with or reject may well differ from your tastes.