Any digital images you purchase will be sized at 10×8″ at 300ppi in JPEG format as standard. This is ideal for printing up to this size or slightly larger (you have more flexibility on size when it comes to canvases, as they’re printed at a lower resolution).
If you know you want to print significantly larger than this, you can upgrade your digital images to larger file sizes, and they’ll be sent to you on a USB drive, just check your gallery pricing for the options here.
One little note though: always back up your USBs, they aren’t the most reliable technology for long-term storage.