Hello There, I have quite a few Poster Sleeves from Selwyns. they don't hold the poster tight, it does slip about a small amount, but not so that there is any damage to the poster. I do store mine upright, the poster sits close to the bottom. They are great for handling posters, but not so good If you have a lot, and want to use them as storage as they take up quite a lot of space. So I only put expensive posters in them usually.
As for Linen backed Quads, they don't fit. You need to order 44"x34" they do make them, but they cost a bit more, as they are cut down from another size.
If you order any, you can save a bit by not getting the gold, or silver corners on, and they do a slightly thinner backing board version which is good enough for home use, as apposed to dealers carrying them back and forth to fairs.
If you need any more info, or pics just let me know.