Try this in a creation expression:

NOTE: you MUST add a radiusPP attribute to your particleShape, and apply this expression to that for this to work as you intend

The BOLD one is the one to use first, as it is what you asked for.
The rest are variants to try for fun

particleShape1.radiusPP = rand(.5,1)

OR

particleShape1.radiusPP = sin(time)

OR

particleShape1.radiusPP = sin(age)

Check out the "EXPRESSIONS" sticky thread in this area as well for more expressions to have fun with, and learn from