I have a smaller force of "large 28mm" Celts (smaller compared to the hordes of "normal 28mm" Celts). It is composed of A&A Miniatures, select Renegade figures (none with the cricket bat swords), and a few odd later Foundry figures. They are a very good match.
I don't have any of the new Crusader Celts, but based on Mark's past ranges, they are likely to be a wee bit smaller than the ones listed above. If you were just looking for compatibility, they'd go fine with the other "large" Celt figures. But if you're looking for something larger than the Copplestone Foundry Caesarians, I suspect you might be disappointed.
Allen