I got my Celtic Cavalry pack the other day. The look pretty pretty sweet, actually. There's a dearth of shields (only one or two per non-horse sprue), but that's easily remedied from the bitz box. They're smaller than things like Reaper horses (i.e. Warlord deathriders), but not so much they couldn't be on the same field.
I have some RAFM "Tribe of the Midnight Sun" horsemen and archers that are pretty close in size. Now I'm wondering whether I should add some RAFM axemen and spears to make a nice quasi-historical army. :-)