Well, this is a rough guide for photo print sizes (R series)
Code Size (in.) Size (mm) Aspect ratio
3R 3½" × 5" 89 × 127 mm 7:10 (0.70)
4R 4" × 6" 102 × 152 mm 2:3 (0.67)
5R 5" × 7" 127 × 178 mm 5:7 (0.71)
6R 6" × 8" 152 × 203 mm 3:4 (0.75)
8R 8" × 10" 203 × 254 mm 4:5 (0.80)
S8R 8" × 12" 203 × 305 mm 2:3 (0.67)
10R 10" × 12" 254 × 305 mm 5:6 (0.83)
S10R 10" × 15" 254 × 381 mm 2:3 (0.67)
11R 11" × 14" 279 × 356 mm 11:14 (0.79)
S11R 11" × 17" 279 × 432 mm 11:17 (0.65)
12R 12" × 15" 305 × 381 mm 4:5 (0.80)
S12R 12" × 18" 305 × 465 mm 2:3 (0.67)
And of course there's the Standard A series sizes. With digital files, you can get your images printed at this size at large format printers. They are considerably cheaper than photo prints. Depending of choice of paper, you can probably get an A3 prints for about S$3 ( S$8 for 8R?). But it'll be on something like an 100 gsm paper and look more like a poster than a photo. Great for school projects, office gathering's promo posters or pinning on office cubicle. And if you have the pixels, you can even print an A0 image for your living room wall etc. Hee hee. Here's the A series sizes.
A Series Formats
size in mm
A0 841 × 1189
A1 594 × 841
A2 420 × 594
A3 297 × 420
A4 210 × 297
A5 148 × 210
A6 105 × 148
A7 74 × 105
A8 52 × 74
A9 37 × 52
A10 26 × 37
Well of course there's the B series sizes (In-between A sizes) and C series sizes (usually Evenlopes sizes for A size papers, an A4 will fit in a C4 envelope, so on and so for)
The ISO 216 paper sizes (The A,B, C series) has some interesting properties. 2 sheets of a smaller sizes will combine to form the next larger sizes (A3 = approx 2 x A4). So it makes enlarging much easier as the aspect ratio of the papers more or less remains the same.(1:√2 aspect ratio). You can read more here:
http://en.wikipedia.org/wiki/ISO_216
(PS: Hee That kinda looked messy... should have made a table instead)