I prewash everything except jelly rolls (which I rarely buy) and it gets washed before it makes it upstairs to my stash. I make 30-40 quilts a year, and don't like working with unwashed fabric. I recently bought a new washer and dryer - the washer has no central agitator. I looked in that large open space and fell in love lol! Right now I have a bolt (15 yards ) of beige sitting on my dryer waiting to be washed.
About the largest I wash at a time is 7 yards of 105" backing. I usually have to take it out of the dryer several times to unroll it so it dries evenly. I find the backing shrinks quite a bit, but it also relaxes and evens out so it loads on the LA much easier.