I have used puddle pushers for pt/pd coating for a number of years and buy them from Bostick and Sullivan. The size would depend on the size of the negative. Usually somewhat larger than the negative so as to fully cover the negative space. If you want clean borders not showing the coating make a mask with rubylith or something that will block the uv light. You could also leave the coated borders and cover with an overmat if that is the presentation you are looking for. That way you can go either way.