A little pressed for time, but a few thoughts...
1. Requiring people to make choices just confuses them and often sets up contests between him and her which end in stand-offs. Never show different sizes of the same piece at the same time. Show them something great with the range of possible choices reduced to YES or NO. I think most of my buyers appreciate such reductions to simplicity. Keep it simple, keep it focused on the image rather than the choices. If you do have a different size of a piece keep it out of sight unless the customer starts hemming and hawing, but try not to get them started on the long and perilous path of choices. BTW I'm an artist, not a short-order chef. Here's my art, here's what I charge.
2. At art fairs your competition will be loaded to the gills with smallish pieces. Make your stuff big and impressive and price competitive and you will be the untouchable king of the hill for that weekend and every other photographer at the show will hate you. Sell "statement pieces" that boldly go where no other piece has gone before, which is to say on the big bare clerestory wall, or above the sofa, or on the reception room wall, or some other featured location just begging for an amazingly impressive slab of art. The client perceives they will be getting a really significant amount of prestige mojo for the price, which justifies the purchase in practical terms as well as aesthetic ones. Nothing appeals to clients like a lot of prestige per $ spent, and that especially applies to business clients who have more bare walls to fill and more $ in their pockets than any body else.
3. And this doesn't mean you have to be a greedy jerk. When I sense a customer is really having a hard time with a decision, I usually let them off the hook with something like "well, you can order one later." I'll never see them again, but I don't have to feel I made somebody spend their last dime or buy something that wasn't truly moving them. And I *almost* always tell clients they can exchange their pieces within a reasonable amount of time, or trade in for full value against something larger, and about 2 or 3 a year take me up on that.
4. And as a general observation, people are almost universally surprised to see that the piece looks smaller all alone on their bare walls than it did at the gallery or art fair surrounded by other pieces.