Available sizes
Once you have chosen to buy a print, a window will appear listing all available sizes and options for that particular photo. 

Cropping
If the size of the selected picture doesn't match the aspect ratio of your chosen print size, cropping handles will appear on the picture thumbnail, allowing you to choose how to best crop an image.

Choosing an aspect ratio
I am a big fan of landscape or even panoramic formats but have generally made my imagines available in their original aspect ratio out-of-camera, which is closer to 4:3. Feel free to choose a different aspect ratio however, they look great for landscape images.

Choosing a size
As a general rule, the bigger the wall space to be filled, the larger you'll want your print to be. Keep in mind that larger prints are meant to be viewed from further away and won't look as good if you are stuck too close to them. So for instance it wouldn't make sense to place a large 30 x 60 inch (76 cm x 76 cm) panorama in a narrow corridor. However it would look great on the longer of a room's two sides. 

The rule of thumb to estimate the minimum viewing distance for your print is 1½ to 2 times the diagonal length of the print. So in the case of our 30 x 60 inch print, you would want to stand at least 8 to 11 feet (2,5 m - 3,5 m) away to fully appreciate it, i.e. a decent size room.

Metric dimensions
Yes, I know, the rest of the world thinks in metric terms, rightfully so. So here is a table of equivalents:

inches  =  centimeters

5 x 7 = 12,70 x 17,78
5 x 10 = 12,70 x 25,40
5 x 15 = 12,70 x 38,10
5 x 20 = 12,70 x 50,80
6 x 18= 15,24 x 45,72
6 x 24= 15,24 x 60,96
8 x 10= 20,32 x 25,40
8 x 12= 20,32 x 30,48
8 x 24= 20,32 x 60,96
8 x 32= 20,32 x 81,28
10 x 20= 12,70 x 50,80
10 x 30= 12,70 x 76,20
10 x 40= 12,70 x 101,60
11 x 14= 27,94 x 35,56
11 x 33= 27,94 x 83,82
12 x 12= 30,48 x 30,48
12 x 18= 30,48 x 45,72
12 x 24= 30,48 x 60,96
12 x 36= 30,48 x 91,44
16 x 16= 40,64 x 40,64
16 x 20= 40,64 x 50,80
16 x 24= 40,64 x 60,96
16 x 48= 40,64 x 121,92
20 x 20= 50,80 x 50,80
20 x 24= 50,80 x 60,96
20 x 30= 50,80 x 76,20
20 x 40= 50,80 x 101,60
20 x 50= 50,80 x 127,00
20 x 60= 50,80 x 152,40
24 x 24= 60,96 x 60,96
24 x 30= 60,96 x 76,20
24 x 36= 60,96 x 91,44
30 x 30= 76,20 x 76,20
30 x 40= 76,20 x 101,60
30 x 45= 76,20 x 114,30
30 x 50= 76,20 x 127,00
30 x 60= 76,20 x 152,40
40 x 50= 101,60 x 127,00
40 x 60= 101,60 x 152,40
40 x 70= 101,60 x 177,80