I’ve run into a dilemma again that, going by Google’s long-standing promise, I should have forgotten about long ago:
The solution was recently given by Big G itself, in its Official Gmail Blog: http://gmailblog.blogspot.ca/2012/11/search-for-emails-by-size-and-more-in.html
This means, for example, to find emails larger than 5MB, you can search for size:5m or larger:5m or to find emails sent over a year ago, older_than:1y.
The full list of Gmail’s advanced search operators
Use Gmail wisely.