The Tantus line of silicone dildos is very firm. We have the Maverick link and due it's combination of size and firmness - we don't use it often - but if your looking for hard and large it may be what you need.
If you're looking to go even larger - the Tantus Hoss link is massive at 2.75" dia and 10.5" long - that's 8.6" of girth - enough to satisfy even the most extreme size queen!
I just purchased a Large Realistic Bent, which is basically the Maverick, but not in vixskin. It seems to be the best large strapon cock. I like the Randy, but it seems to be an inch too short, even though it is thicker (which is a plus!)
What type of dildo are you looking for? Do you prefer one to have balls or not? Do you crave a large or a very large, based on thickness?
Maverick, Outlaw, General or some of the Vamp Silicone (not the Tantus Vamp) models are probably your best choices. There's a company called SquarePeg that makes the really big stuff if you're into that.