Reviving this because the deadline has passed and a few people here were facing a mountain. OneRealOchreHiker, 220,000 unread at 21 GB is very doable, just not one email at a time.
The trick is that Yahoo can delete far more than the page you're looking at. On the website (not the app), search for a sender you never read, a shop or a newsletter, tick the box at the top of the list, and a small "Select all that match" link appears. Click that and it selects every message from them, thousands at once, then Delete. Ten or fifteen senders like that usually account for most of the pile. For old stuff wholesale, search before:2021 and do the same.
Two things that catch everyone out: deleting only moves mail to Trash, and Trash still counts, so empty Trash and Spam at the end. And the storage figure on screen is cached, so it looks like nothing happened until you do a hard refresh (Ctrl+Shift+R).
Full disclosure, and HQ please cut this line if it's not allowed: I got so fed up doing this for my own family that I built a little desktop program that does the sorting-by-sender-and-size part for you. Not linking it here. Happy to answer questions about the manual method regardless.