I have used a 6,7,8wt. flyrods for steelhead, and I now have two 8wt that I use almost exclusively. A lot depends on the water you will fish, the fish that you expect to catch, etc... My favorite all around steelhead rod is an 9'6" 8wt. The longer rods will help you mend and control your line on the water and can help you cast a little farther. Best thing to do is to go and cast a few rods so that you can feel the difference between a slow action rod to a fast action, etc....find out where your wish list meets your wallet.
PS my first steelhead fly rod was a Fenwick glass 7wt. (about $70) and I have caught numerous trout, steelhead, and salmon on the old beast. I used it about a year before stepping up.
Good luck!