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