The size is on the sharable links. On the popup window where you can pick links of different formats (direct link, BB code, etc.) in the bottom right it has different sizes available which change the link structure. The idea looks like they want the original and you can choose to share it at smaller image sizes.
Personally I don't see a need to limit image sizes, imgur limits are large enough it usually won't trip up most users and most modern forums will automatically limit the displayed size. If I'm sharing something publicly I share the full resolution. I like airplane photos with enough resolution that I can read the stencils. Sharing low res images limits their usefulness. It's one of the things that bug me with a lot of flickr galleries. Some guys post awesome reference image galleries, but the resolution is awful and then they disable downloading.