There's also the large construct that can be created by an Enchanter.
That said, I'd be fine with the use of somewhat larger models for regular soldiers too, assuming somebody wasn't very clearly doing it to get an advantage in some way or another. By that, I'm thinking of the larger figures not just being missile troops (who could benefit from extra height to shoot at figures otherwise obscured from view) or to for example have an extra large Knight with items making it extra tough, and then hide several softer figures behind it or so. In other circumstances, being larger is probably just a downside in Frostgrave really, so just use whatever nice figures you happen to have. Getting cool stuff on the table and having some fun is all the game should be about!