I am running LR 4.1 on a new Dell XPS. When I try to set page size in the Print Module, the only driver I have is the XPS one with pre-sized sheets (A, B, C, etc). I want to set a 16x20 size for a custom job. Do I need to download a 3rd party Printer Driver to do this? I do not own a printer at this time.
Paper size it is always controlled in the printer driver. If you don't have a printer then there isn't much you can do unless you want to download a driver for some printer that might meet your needs. The other alternative would be to crop the image to the appropriate aspect ratio that you want to print (16 x 20 would use the 4 x 5 aspect ratio in Lightroom) and then export a JPEG file to send to a lab to be printed.