Hi, welcome to the Apple Users Community.
Please first check to see that you are not in full screen or tile window mode by moving your mouse to the green circle in the top left of your new message window. If you cannot see the green circle hit the escape key. This will bring you out of full screen or tile window mode.
Then try hovering your mouse cursor on the right side of a new email window and you should see a cursor change that looks like this ⬌ . This will allow you to adjust the width of you new email window. When you have got the window to the size you are happy with close the email without making any edits to it. Then close/quit Mail. Reopen the Mail App and when you open a new mail it should always open at the same size you made before closing the software. You have now set a default.
Best of luck