Hi mahoopes2,
Welcome to Apple Support Communities. We understand that you're having an issue with the size of your Mail window on your MacBook Air. We'd like to help.
From what you've described, it sounds like the Mail app is showing as longer than expected. Have you tried to
go to the left or right side of the mail app and manually make it smaller? You can do that by following these steps:
- Hover your mouse over at the edge of the page until you see the double arrow cursor
- Click the edge of the page and move in the direction of the other end of the screen to make is smaller. T
This can make your Mail app more manageable. Once that is back to normal size, are you able to click the green button then?
Let us know and we'll go from there.
Take care!