This was becoming an issue as I keep a local backup of my Gmail account using IMAP Backup. And while you can find attachments in Gmail using search modifiers
filename:zip, you can’t search or sort on attachment size.
The solution: the IMAPsize freeware Windows IMAP utility found via this blog post from Amit Agarwal. IMAPsize’s speciality is searching your Gmail account for folders (Gmail labels) and messages based on attachment size.
I quickly found and deleted my large attachments using IMAPsize. Backup problem solved. Recommended.