Here's how I'd do it:
Step 1
Select 19px diameter paint brush.
Step 2
Click the "Toggle the Brushes palette" button, which I have circled in red here:
Step 3
Change "Size Jitter" to 100 percent under "Shape Dynamics", like so:
Step 4
Under "Scattering" change the "Scatter" setting to around 400 percent, like so:
After you've completed the above, select your colour and start drawing with it. I recommend you play around with the settings to get what's good for you.