I purchase all my meat from Costco as they seem to have the best quality and Require the least amount of fat trimming and prep. They are massive St Louis style racks compared to most grocery stores as they done cut off the top layer of meat to sell separately. They weigh in at an average of 3.2 pounds for an 11 bone rack. I can feed 3+ from a rack as most people will only have 2-3 ribs.
Anyway, I won't waste my time with the smaller racks as they just take up too much rack space on the pit for the little meat you get out of it. So yes, I always get the biggest ribs I can find.
I think the thing to remember is to get whatever works for your situation but remember that they cook differently and may require a little different cooking time so sticking with the same general size allows you to build a lot of confidence in your cook.