i see what you mean. imsanity goes to work on the image after wordpress has already done it's security checks so the size error happens before the plugin has access to the image.
it looks like it is feasible to do that using a different plugin hook, but it would require changes to the way imsanity works. i think it's a good idea, though, i'll put that on the list.
if anybody has time to contribute, i'd be happy to add it to the code. right now imsanity hooks into "wp_handle_upload" where the file has already been accepted by wordpress and the file type has been determined. But it looks like it could hook into "wp_handle_upload_prefilter" instead, in which case imsanity will have to figure out the file type and do error checking.