You can choose between:
Small: 1024 x 768 pixels
Normal: 1536 x 1152 pixels
Large: 2048 x 1536 pixels
Extra large: 2560 x 1920 pixels
Larger photo sizes imply sharper photos, but they also require more storage space.
This setting is only relevant for the photos stored in Topo GPS. Photos that are saved in your photo library are always saved in the original resolution.