I may not be understanding your problem correctly but ...

I usually have the New Mail Folder and Mail Folders windows displayed side by side equally sharing the screen. Reading a new message and other activities often changes this arrangement so I'll close all but those two original windows, make sure the New Mail Folder is the active window, then select Window/Tile from the menu bar which arranges both windows per my preference. I always make sure the screen is laid out this way when I exit the program as Pegasus restores the layout as it was set up when the program is closed. 

I must have hit some key or done something and the result is very annoying!

Every time I check the New Message item, the message window occupies the whole screen. If I ask it to RESTORE to my smaller window, the result is that it minimises to the bottom corner. Thereafter everything (such as new message list) opens to full screen and I then have to RESTORE (and it does that satisfactorily).

What have I done and what must I do to rectify?

Charles Ellis

I have had instances where a window is reduced to a small window at the bottom left of the screen, as if it were minimized but it is still windowed.  I have to stretch it back to the size I want it.  The stretching of the window resets its size.

Also, you may want to reset the default folder layout.  Options are in Folder >  Default size and layout.