Just as a note:
First run of Gimp, when there is no Gimp 3 User profile (as with a new install) does try and migrate settings from a previous version. There is a bug report here about Gimp 2.10 / 3.0.4 Not really about window size but that might come into it.
https://github.com/GNOME/gimp/commit/204...a47fe4a800
Always better to disable your old Gimp User profile - rename or move to another folder for a new Gimp 3 install.
First run of Gimp, when there is no Gimp 3 User profile (as with a new install) does try and migrate settings from a previous version. There is a bug report here about Gimp 2.10 / 3.0.4 Not really about window size but that might come into it.
https://github.com/GNOME/gimp/commit/204...a47fe4a800
Always better to disable your old Gimp User profile - rename or move to another folder for a new Gimp 3 install.