Silver Ticket is quite unlikely to have a specific size that isn't standard. Some companies do offer custom sizes and don't charge more for them, but they will cost more from the start. So, looking at Seymour AV, they will custom make any size you want. But, the price will already be a lot more than Silver Ticket or Elite. So, there's a tradeoff there. I wouldn't say I've seen a huge difference from one white screen to another. Minimal differences, yet, but not major ones. So, shopping on price is just fine for standard white screens.
You MAY want to consider if they offer a narrow frame option. This may gain you a few more inches of screen size.