Some of the QRF Celts are based on the old moulds from the 80s and others are only a few years old. I don't have any experience of the newer figures but do have some from the older moulds (mostly the naked figures).
I only have a couple of Essex Gauls and these seem to be on the small side for Essex. Compared to the QRF I have, they are quite a bit smaller. I have mixed my QRF figures with Corvus Belli, Xyston and Forged in Battle and they work well, being on average between CB and Xyston in height.
The figures based on old masters are still good quality. I think they remoulded them when they redid the range, so the castings are crisp enough. They hold their own with figures sculpted far more recently.
The older sculpts are mostly in packs 14, 17, 18, 19, 20, 21. They are perhaps a touch larger than the newer sculpts – the musician pack has an older sculpt (playing the carnyx) and a newer sculpt – the older one looks a little bigger but not unreasonably so.
steps into the arena with two gladiators from 
