Lee over at Napoleonic Wargames Project has asked me a couple of times how different manufacturers of Cavalry and Artillery compare. I finally had a chance today to take a few snaps, sorry I do not have fancy calibrated back drops, but I believe a sense of the figure size is more important than actual sizes.
I am not as picky as some and I actually like having different manufacturers in my armies, my infantry units have figures from 6 different manufacturers and I am happy in my own way with them all. I find that it is difficult to mix different manufacturers in the same battalion, but in respect to command figures you sometimes have to do this. I also generally like to use the same manufacturer in the same brigade when wargaming, but this is not really for the look, but more a question of easy identification, ie. the Foundry's are the conscript's etc. etc.
Recently Lee has asked me about both artillery and cavalry, he had an attack of the "wobbles" over the Victrix Plastic artillery, and I think I can understand why. The cannon carriages are really quite diminutive, I really can not comment on the figures. Fortunately he seems to have got over it and he recently has done some great work.
Anyway back to the point of the post. I do not have all the manufacturers but I do have some and have taken some photos. Here are some cavalry, I believe that these are all pretty similar in size, aside from the Foundry, which are definitely smaller. I would not mix the other figures in the same unit, but I believe they would be fine in the same brigade. In respect to the Foundry cavalry, I have 2 units and a 3rd unit to paint for Lepic's Imperial Guard Cavalry brigade to add to 2 regiments of Front Rank cavalry. It was clearly a mistake to do it this way and I would avoid using Foundry cavalry unless you plan to do your whole army in Foundry (Front Rank infantry seem to tower over Foundry cavalry). Nothing wrong with Foundry figures, but they do not mix well with some other manufacturers.