Resizing Email Response Window

• If you can’t access the green resize button because it is off screen, just click on the window you want resized and then navigate to the Window menu and pull down to ‘Zoom’. This will automatically resize the active window to fit on the screen.
• Finally, another option is to grab any portion of the window you can and hold down OPTION+SHIFT keys and drag, which will resize the window in any direction

Another option is to temporarily switch the display resolution to smaller than you’d typically use, since that resizes onscreen windows to fit the new screen resolution. This is possible from the  Apple menu > System Preferences > Display section of settings.

https://osxdaily.com/2010/07/05/how-to-resize-a-window-that-is-too-big-or-off-screen-in-mac-os/

And a second click on the Green dot just brings it back to still too large?

Safe Boot, (holding Shift key down at startup), does the problem occur in Safe Mode? Could take 10 minutes.

Safe mode attempts to repair Disks & clears lots of caches & loads safe Drivers, & prevents loading of 3rd party extensions, so if Safe Mode works try again in regular boot.

Ah, thanks, take care my friend! :)

Do you mean FullScreen or just a big Window?

Can you grab the lower right corner and drag it up & inward?

Does the Window have the Green, Yellow & red dots at top left? And Green dot does nothing?

Hmmm, Mojave 10.14.6 is later OS than High Sierra 10.13.6

You mean OS versions like Catalina & Big Sur?