The first change seems fine from the image. It seems good to not force a minimum size unless truly needed (or something really small), and the default size of the dialog needn't/shouldn't change.
For the other stuff, perhaps it could be made more responsive so that once the dialog gets too small to fit, the contents could start adapting. I think GTK+ 3 has some new widgets to help with this (ex. [GtkFlowBox](https://developer.gnome.org/gtk3/stable/GtkFlowBox.html#gtk-flow-box-set-col...)). It would be more complicated code-wise but it would allow your use case without negatively affecting those of us with 4k screens and dual monitors or whatever.