Hi Giovanni, many thanks for getting back to me. Apparently after painter 2015 the bracket keys don't work for resizing brushes. The way to do it now is to hold the alt and command keys and then put the pen on the screen and move it outwards, this enlarges the brush. I have tried setting up a shortcut with just the alt and command key, thinking that perhaps I could then use the stylus as now but it doesn't work.
Unless you can come up with a suggestion I'll continue to use the alt, command stylus combination.
Best wishes, Teresa