All images shown on the general photo pages fit within a 1024pixel-square area. If they are bigger than that when uploaded, the site software will reduce them to fit into that frame. If they are smaller than 640px, the moderator is likely to request a larger version.
Optionally, you can upload a larger version of the same photo and license that larger version with the same Creative Commons licence (unlimited free re-use by anyone for any purpose as long as credit is given) as the main image. To do this, select the largest image size that you are happy to contribute when prompted by the submission dialogue after uploading your photo in step 1. Geograph will produce the standard 1024px image as well as this higher-resolution one according to your request.
Site users can access these larger images by clicking on 'more sizes' above the main photo on the photo page.