If you want to have a nice clean board (or maybe you're doing some repair) you can use this stiff-bristled brush without worries. Some people like to drip a little rubbing alcohol on their PCBs after soldering and then scrubbing to remove rosin/flux. Finally, you have the right tool for the job.
About 7" long from end to end, scrubby part is 1.5" x 0.5" with 0.5" bristles.
Adafruit Product Page: https://www.adafruit.com/products/1209