According to English Wikipedia, most stubs contain two to seven sentences (50–150 words). Given our estimation of five bytes per word, that's a upper limit of 2500 bytes, which would correspond to the beginning of the easy range. We could multiply by 4 to get the medium and by 4 again for hard.
Following the same pattern for section but starting at 1000 (4 sentences), we'd have 4000 for medium and 12000 for hard.