Deleting Content

Hello, I have a Premium Tuta plan but have received a message when I log in:

"Your mailbox has nearly reached the storage limit. Please free some storage by deleting content from your mailbox or upgrade to a larger plan."

What is the most efficient way to delete content from my mailbox? I see that I can sort by emails with attachments, but is there any way to filter emails by size? The lack of clarity about this or a storage management tool is concerning. Any suggestions?