Back in film days, having your pictures printed was a necessity, but in the digital age, sometimes you only store photos in your computer, cellphone, CD/DVDs and memory cards.
My film days are over but sometimes I still don't remember the dimensions of photo print sizes commonly referred as 3R, 4R, 5R and 8R so here's a list of the picture sizes for your information:
3R (3.5 x 5 inch)
4R (4 x 6 inch)
5R (5 x 7 inch)
8R (8 x 10 inch)
8SR (8 x 12 inch)
other sizes are 12 x 18 inch, 16 x 24 inch and 20 x 30 inch.
One way to remember how long and wide a picture size is, sort of a mnemonic, is that the number corresponds to the length in inches of the shorter side, the the longer side two inches longer than the shorter side, except for 3R picture size with only 1.5 inch difference.
These photo print sizes were derived from 35mm film format (36 x 24mm) with aspect ratio of 3:2 like my DSLR camera and large-format (4x5) film format.
The standard ratio for 35mm negatives correspond directly to 4 x 6 inch (4R), 8 x 12 inch (S8R) so my usual choice in printing is 4R and 8x12 to avoid cropping and resampling.
This is a problem with developing or printing pictures taken using point-and-shoot cameras with 4:3 aspect ratio, you need to crop or resample your picture so that no pixel will be left out of the printed photo. If you don't do this, some part of your picture my be cropped when printed. To make sure the whole picture is printed, resample your picture using Photoshop or other photo editing software without noticeable distortion in the resulting photo otherwise you really need to crop.
You know where to have your pictures printed cheaply? Hidalgo St in Quiapo if you can get there :P