The UI was overhauled in the 3.0 update on March. The new documentation says changing brush size is fairly easy: https://testing.docs.gimp.org/3.0/tr/gimp-using-variable-size-brush.html
All brushes have a variable size that can be changed.
You can change the brush size in several ways:
By using the default shortcut keys for changing a tool’s size:
Decrease size by 1: [
Increase size by 1: ]
Decrease size by 10: {
Increase size by 10: }
By using the default mouse scrollwheel actions for changing a tool’s size:
Decrease size by 1: Ctrl+Alt+Scrollwheel Down
Increase size by 1: Ctrl+Alt+Scrollwheel Up
And there’s a huge difference between fifty degrees below zero and absolute zero. But both are lethal to humans if exposed to it for a night.