Gmail now supports searching by size: http://support.google.com/mail/bin/answer.py?hl=en&answer=7190

The new operators are:

- `size:` — Search for messages larger than the specified size in bytes
 - Example: `size:1000000` 
 - Meaning: All messages larger than 1MB (1,000,000 bytes) in size.
- `larger:` and `smaller:` — Similar to `size:` but allows abbreviations for numbers
 - Example: `larger:10M`
 - Meaning: All messages of at least 10M bytes (10,000,000 bytes) in size.

You can now search for emails of that size and delete any to free up space.