A4 photo paper is by far the most popular photo paper size (A3 comes second). A4 photo paper provides a ready-made nice sized print; you can print a number of smaller images on a sheet of A4 photo paper and then trim them out afterwards. A4 photo paper is available in a range of weights from 104-255gsm. When choosing between matt and glossy A4 photo paper (or somewhere in the middle), go for the look and feel you like best.