Maybe I don't fully understand your problem but the main window size is adjusted automatically.
It is by design. It is a good thing, not a bad thing.
There is an option in
Environment ->
Window settings:
"
Automatically adjust IDE main window height"
but unchecking it does not do much good. You get a bigger main window but the extra space is not used for anything. It is wasted.
The option was only useful to get around a bug with some window managers where the automatic resize did not work.
The main window width can be changed manually. Only height is adjusted automatically.
I would like to know: what is the actual problem you try to solve?
[Edit]
Is your problem related to this one?
http://bugs.freepascal.org/view.php?id=31352It looks like the width cannot be stretched all the way to screen's right edge. Few mm are missing.
I don't have such a bug here. Can somebody reproduce?
Maybe some window managers report a wrong max width or something. It should be debugged.