#1 don't offer the same art smaller. the buyer will odds are, buy it for smaller.
That said, as a buyer of art, I do not buy posters. Ever. Never. Ever. I hate them. I have no wallspace in my house, and someone said I could keep the 8.5x11 posters in a binder- which I do- but who honestly goes through that? Not me. Instead I buy the shorter 7" and below category. I have 3 narrow ones that are like 15x5 that are on my wall from one artist, primarily because one can fit in this "dead space" of my wall where I wanted art. The only larger than 8.5x11 posters I have are framed - I have one from Shilin (deviantART) framed, one that's the cool invisible guy from GAIAonline (the actual artist was at SakuraCon selling her prints and i was soooo happy to watch the stream of her doing it, and made a point to buy it. it's my favorite), and then an official promo poster for KH2 (also framed).
That alllll said, from what I've seen and talked to people at anime cons, you either get the bigger sizes, the medium sizes, and try not to carry the 8.5x11's since they either won't move, or be so cheap that they're the only thing that moves. If you're on facebook I recommend looking up the Artist Alley Network International group since they have lots of feedback about stuff like this. They have a discord too. Pretty sure this is the group, I'm not able to get on my fb right now to see: https://www.facebook.com/groups/artistalleynetworkinternational/
Again, I'm the oddball buyer that I'm not trying to be sold to. Your goal is to sell to people who already want posters, and with that, follow that rule at the top of my post. Offer two sizes, a big and a 20% smaller, but do not offer the same art smaller than 20% of your biggest size because of issue.
And a side note, but also figure out how to effectively display, transport the display, and transport the actual prints without damaging them. There's sites who offer bulk bags for displays (I use them and they're awesome as heck).