My Gmail was nearing the 15GB free tier limit. I tried a few things to try to pinpoint the largest emails to delete, from searching for large attachments to writing code to count emails by sender. The problem was there were no large emails, just many thousands of often daily marketing emails. Identifying and deleting emails from frequent senders proved quite labor intensive.

In the end, the simplest solution was the most effective. I searched for the word "unsubscribe", selected all results across all result pages, and deleted them. I searched my Trash for a few senders I wanted to make sure not to delete email from, moved them back to the inbox, and I was done.

Easy.