When working with different image sizes brushes that feels just right for one image can be very large if you work in a small image and tiny if you work with a large one. Sometimes you have to constantly scale your brushes to work with them.

I'd like a general master brush scale. When working on small for web images you turn the size down and when you're painting for print you turn it up. The individual brush scales then scales relative to this master rather than in pixels.
It would perhaps be a good idea to keep the current pixel sizes but add an option to have brush sizes in percent of a globally given value.

Sounds useful! I could use it for something like pixel art where you want your brushes to all be integer increments. I am aware holding control(iirc) snaps to integers with the size bar but when resizing with shift its minimum size is 0.10