If you don’t trust Auto-Save, of it it stops working like it did for me, set up Keyboard Maestro to also hit ⌘ S on other shortcuts you use regularly.
I have several common shortcuts doing this, here it is with the Return key for example:
I also have it saving every 3 minutes, just in case!
On a Mac, select the Brush tool, hold ctrl + alt, then click and drag up and down to change hardness and left and right to change size.
You can also press [ and ] to change size, and shift + [ and ] to change hardness.