Mail app on my iMac has started to open as a full-size window. Can't grab the bottom right corner to drag the size required. I restarted and it opened to normal size but on opening an email the window size jumped to filling the iMac screen.(27 inch, mid 2010). Using Mac OS High Sierra, v 10.13.6. Are there key combinations for resizing or settings to change?
It sounds like you've put it into full screen mode. You can either move your mouse pointer to the top left of the screen so the three coloured buttons appear and press the green one (you must have pressed that to enter full screen - it toggles) or you can press the escape key.
Roy Rainford 