The only approved size that fits all of the prison criteria is what InmateAid prints it's pictures on - 4" x 6" glossy photo paper. Polaroid-type paper is definitely not allowed or any multi-layered paper. Also, computer generated photos are not allowed in the majority of facilities.
https://www.inmateaid.com/ask-the-inmate/what-size-pictures-can-you-send-to-a-inmate#answer
Accepted Answer Date Created: December 18,2014