Thanks, Brian. I never considered the fact that after a poster was washed etc, that it may retain a very slightly larger size, once it had dried. So it sounds like the gel sizing must "help" the paper to hold that slightly larger size.
Again, really interesting.
(And then, normally, (with traditional linen backing), that bit of a linen border that is left, even if no more than 1/4-1/2" all the way around, in some cases).