Have media open in current window size

[advice2020]

Had a question about allowing media to open in current windows size.

Linux Debian based distro / GTK based Desktop Environment
MPV 0.35.1-4
FS = full screen

I am using the “UMPV” script, set to “replace”
I have MPV set to open all media in FS “fs=yes”

I would like to be able if I want, to adjust the FS size window to a specific windowed size. Then being that I am using UMPV I would like to have any media items loaded AFTER changing MPV to windowed to open in that specific windows current size, not stretched I want to keep the aspect-ratio, just fitting into that windows size and adding any black padding if needed.

If I keep my MPV FS, using UMPV I would want any media loaded AFTER to remain FS. This is how things currently work, I am just mentioning just in case solution for above issue causes this to no longer work this way.

I do not think I am looking for options like “autofit” & “geometry”
I want to use the random size I decide to make the window.
I tried auto-window-resize=no in my mpv.conf but this did not work.
The FS overrides with each media item loaded.
Would I have to add something to the UMPV script file?

Does anyone know if this is possible?
Thank You

[advice2020]

Still interested if anyone has any ideas