The standard that most facilities accept is a 4 x 6 inch photo, which is the same size as a standard print you would get from any photo lab. That format is widely recognized and approved across jails and prisons because it is easy to inspect and does not create concealment concerns the way larger or unusually formatted prints might.
InmateAid prints photos on glossy photo stock at exactly that size, printed edge to edge. These are not cheap reproductions on flimsy paper. They are real photo prints that hold up and look the way a photo is supposed to look, which matters when you are sending something meant to be kept and looked at over months or years.
A few things worth knowing before you send. Some facilities have additional restrictions beyond size, including limits on the number of photos per envelope, prohibitions on certain types of content, or requirements that photos come from an approved vendor rather than being printed at home and mailed yourself. Sending through InmateAid addresses that last concern automatically because the platform is designed around facility mail requirements and the envelopes are recognizable to mailroom staff.
If you want to send photos directly without using a service, confirm the facility's specific policy first. A 4 x 6 glossy print is the safe standard, but calling ahead to verify never hurts. If you go through InmateAid, the sizing and format questions are already handled for you.