The default window size ought to be what you last selected. In other words your resized window dimensions should "stick".
Did you create any custom Mail signatures: Create and use email signatures in Mail on Mac - Apple Support. If so, try deleting them, and then quit Mail. Then, hold a Shift key while launching Mail from Applications or the Dock.
That action will open Mail in windowed mode (not full screen) with no messages selected, and is discussed here: If Mail unexpectedly quits when viewing certain messages - Apple Support.
Although that particular problem is not addressed, review Troubleshoot email problems in Mail on Mac. Malformed email signatures have been known to cause bizarre Mail behavior.
For a very remote possibility read Resolve issues caused by changing the permissions of items in your home folder. I think you'd have other problems if that were the case though.