I have ordered from them also. Actually my local plant club NEAPS did a big order through them.. Your best bet is to call and talk to Jim(I think it was Jim) personally. Tell him what you want and what its for and he does a nice job. Definately generous with his measurments and quantity.
You definately want sandblasted stuff and if you tell him its for an aquarium he will pick through his long dryed stuff to get well seasoned stuff. This is best. Soak it for a couple weeks or weight it down int he tank and you'll be all set. The branchy stuff of smaller diameters is best. A few bigger diameter pieces are nice as you do want to use an assortment of sizes to avoid a monocromatic appearance. Thats why the thinner branchy stuff is best, looks more natural in the aquarium.
Also, mention to him that youwoul dlike to avoid cuts of branches to fit size requirments. Obviously there will be a cut at the butt end, which is th epart youwill probaby have in the back or buried in substrate. Then, when you get the wood, if you need to remove a portion or a branch break it off as it will look most natural, especially once the wood gets that aquarium "patina" in a few weeks of submersion.
As for lengths, you should get some 24", some 18" and maybe even some 12" Remember you can always break or cut a piece down but you can never make it bigger
Good luck and be sure to post pics once youset up the tank.