select your particles... go to the attribute editor (ctrl+A)... go to "add dynamic attributes" and pick general. Then klik on the tab
article. From this list select RadiusPP (radius per particle) and click Add.
In your attribute editor, click "Per particle (array) attributes" and you will see RadiusPP in the list.
Now rightclick and select: creation expression
Play with the numbers until you have the result you like.