Size is an old English word for glue. Gold Size is the glue used for applying bronze powders and metal leaf to surfaces.
This particular size is water based and has the advantage of staying sticky..
Paint it on thinly, allow 5 minutes to dry. When dry it becomes clear and then is the time to apply either bronze powders or metal leaf. Any unused areas may be waxed, varnished or washed, making them no longer sticky.