Yes, this brush icon size thing and many other is just personal things.

Myself I at the moment prefer these small icons.. 48x48.
And that is of course only because I use my own brushes and overall,
I do know what is what.

I use there also modified "Color History" that shows not 5 but 8 last used colors.

And "Color Selector" shows me already Triadic and Complimentary colors.
So I do not have Select these harmonies when I open Color selector.

These are mine "personal" preferences, so I do not say that these
should be a defaults. Same is true about that "gtk_menu".