I'm on a Mac as well but I don't see the single window icon at the top right of the FMP menu bar, so based on a limited sample of 1 that is not the issue.
Have you tried to get your cursor to the edge and resize the window? perhaps try clicking the minimize to last- maximize to last button (not sure what the correct name for it is - the green one in any case)? I just tried on my system creating 2 different sizes, quitting and restarting and they reopened to the size when before I quit.
Filemaker Pro --> Preferences --> General --> click on "Reset" at the right of "Reset dialog sizes and positions" --> confirm by clicking on "Yes" --> close clicking on "OK", then press cmd-shift-S.
Do you see a change ?
Yes, that worked. Thanks for the tip.
Your technique worked also. Many thanks for the tip.