I don't think size matters too much once you have them all painted and ranked up- though, I do admit I usually stick to one range.
Where I have strayed into using virtually anything to hand the results have been pretty good,
Rather than hijack then thread I'll stick a link here for you to check the results of mixing, Wargames Foundry (oldest), Wargames Foundry (newest), Gripping Beast (oldest), Gripping Beast (newest), Saxon Miniatures (Now Warlord Games), Drabant miniatures (the largest), Footsore Miniatures (oldest), Footsore Miniatures (newest) and probably other stuff that I've forgotten about!
https://justaddwater-bedford.blogspot.com/2022/03/all-my-anglo-danish-units-so-far.html