Welcome to PA, no you don't have to size the back first. I spread an even layer of glue on the back, then lay the linen on top of it use your fingers to work the
glue into the fabric and smooth it down. I like to work from the grip to the tip. Then I go ove the top with another thin coat of glue. I leave the linen oversized and trim
it with a razor knife once the glue sets up but isn't hardened all the way yet.