deckm,
If you have things like "custom" or "PostScript Custom Page Size" in your list of paper sizes, they are not coming from PrintShop Mail, but from the driver. This means that for PrintShop Mail, it's just another paper size, and it doesn't recognise it as a "custom" size. You will need to click on Advanced, and find the place where you can change your paper size(which is different on every drivers). Once you change it there, the custom size should reflect the change.
Regards,
Raphaël Lalonde Lefebvre