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
They wanted "first Sunday of Spring" but defined using their calendar, but that calendar doesn't mesh perfectly with our calendar (and has leap months every few years), so converting to Gregorian makes it appear to move around.