Use Thunderbird to easily find large Gmail email messages (you can sort by size). You must do this using Gmail's "All Mail" folder in Thunderbird.

Identify a Local Thunderbird folder to keep a copy of the message if desired.

Right-click the email in Thunderbird (Gmail "All Mail") and choose "Copy to [folder]"

Then delete the message from All Mail. It will be moved to Trash and you can empty trash inside the Gmail web account.