I'd also add, look for one that's roughly the same thickness across the cut - I saw a bunch at the store that we thick on one end, and thinner on the other. I wouldn't think that such a beast wouldn't cook evenly
In addition, I don't think I'd buy one smaller than 2 - 2.5 lbs, as I would be worried that it would dry out.
All in all, good luck with the pastrami! In fact, I'm enjoying the last of my Pastrami right now - going to have to make another soon!