I prefer to print my pictures on Alu-Dibond, with or without a butler finish, depending on the picture. Usually, pictures with a metallic glaze or big white surfaces will get printed on Dibond plates with butler finish.
With the butler finish plates (the alu is brushed), white is not printed. So, where white should be, you can see the metallic, shiny and brushed texture of the aluminium. I love it!
Pick a size that fits
Depending on the wall you will display the picture on, the viewing distance and off course your own personal taste, pick a size that fits.
Follow the link below to get an idea of how several standard formats look like in a generic room.