Are you talking about the plastic stuff they are selling now, or the older metal ones?
The metal ones were a good 28mm+
The plastic ones may vary some, but I have several of the human-like ones and they are barely 25mm, maybe a touch more, and fairly anemic too. Their bases are molded on and quite thin compared to a plastic slotted base or a Clix base, maybe half that height on average and usually just wide enough for the fig to stand up properly.
While they do fit fairly well with most of the 28-30mm metal stuff out there as they were intended to do, they will look a little smaller and thinner than the average metal. Their monsters are a bit bigger from what I've seen, but I'm no expert.
If you are looking for plastic, take a look at ebay or companies like
www.pjcc.com for their mage knight and heroclix singles. The quality varies, but most of them are superior to the D&D stuff IMO. The latest set of the D&D plastic looks like it might be better at a glance, but I havent seen any up close yet.