When first launching the settings dialog for trash/waste-bin, "Size" is checked, but the box for setting the size is disabled. You have to toggle "Size" to activate it.
A possibly relevant merge request was started @ https://invent.kde.org/network/kio-extras/-/merge_requests/353
Git commit 342ae3694f08d7fddd71d527800da9c00387dcae by Nate Graham, on behalf of Paul Worrall. Committed on 08/06/2024 at 19:31. Pushed by ngraham into branch 'master'. Correct trash kcm dialog initialization The dialog is first displayed with (for instance) Size checked but the Limit to spin box disabled. This is because the dialog box elements were not connected up at the time the useTypeChanged method is called FIXED-IN: 24.08.0 M +2 -1 kcms/trash/kcmtrash.cpp https://invent.kde.org/network/kio-extras/-/commit/342ae3694f08d7fddd71d527800da9c00387dcae
*** Bug 478052 has been marked as a duplicate of this bug. ***