Dirk, I would say they all the main manufacturers are broadly compatible in size, as you can see from my Game of Thrones thread, where I've intermixed parts from Perry, Fireforge, Warlord and Gripping Beast. The original Perry Wars of the Roses figures are a very tiny bit smaller in stature than the others, but the more recent sets (Foot Knights) are taller, and the Agincourt to Orleans set is taller again. But all the parts can be intermixed perfectly well. You wouldn't really notice any difference in sizes.
What is different, is the style of the figures. For my money:
The Perry medievals are far out in front for quality, number and variety of components per box, and 'naturalness' of poses and anatomy. They are just a superior product to all the rest.
Fireforge - I don't dislike the figures. The infantry bodies are pretty good and very versatile for generic medieval types. The shields and assorted great helms are all very nice, as are the crossbows and most of the cloaks. What I don't like about Fireforge medievals are the cartoony faces under the kettle hats and open faced helmets, the GW-style ludicrously outsized hand weapons, and the horses heads which look like skulls.
Gripping Beast (Vikings, Saxons, etc) - I don't dislike these figures either, although they again tend towards the cartoony in the heads department. And I'm not keen on how they've done some of the mail - the sleeves in particular can look a bit odd. Overall though, pretty good.
Warlord - to be fair I've only got the Celts set. Again, slightly cartoony, and the arms come joined to the torsos, so a bit less flexibility. What I notice though, is that wherever I've used components from this set - they actually look really good on the finished product, which I think tells you something.
But they're all pretty easily interchangeable.