You can successfully scale down brush sizes, but unfortunately if you scale them up you will lose sharpness and definition. The only way I can think of that
might work is to create your whole image in proportion to the brushes you want to use, and then scale the finished image up to the size you want it. You can do this with
Image >
Image Size... but the results can be very variable. I have had good results with a plugin from Alien Skin called
Blow Up:
Blow Up
It's maybe a bit pricey ($199), but you can download a fully-functioning 30-day demo version which will do the job if you've only got the one image you need enlarging.