As someone who buys all art off Etsy, please offer both. My printer is not good. I have made the mistake of buying the download and been super pissed. I don’t understand buying the download if I still have to go to staples or something and get the right paper and have it printed a certain way. Sorry for the rant but I’m a big fan of buying the physical print and so many people don’t sell them anymore!!
I usually offer PDFs at 8×10 (U.S. standard frame size) or 8.5×11 (U.S. letter size). Let’s assume we’re starting with a PDF in one of those sizes, and let’s assume you aren’t using Photoshop (highly recommended!) or a comparable file-handling app. Printing the PDF from default programs (Reader, Preview, Word) should give you a dialogue box that allows you to adjust sizing by percentage.