Thanks for the suggestions, alas the current version of the software does not allow us to impose the limits as they will affected ALL files, not just JPG. This would stop people uploading MP3s over 1Mb which isn't our issue at the moment. Also - it won't remove old files that are too big. Software will be upgraded in the next couple of months which will allow us to prevent the issue reoccuring - for now though, it's file culling.
FWIW i've been targeting MOSTLY files that are 4MB and over a year old but also a few JPGs that are 6MB+ and more recent.