I thought it had some kind of instruction. Okay, so, when you type ~reroll, follow it with the SPECIAL you want in the order it is spelled.
example:
~reroll 5 5 5 5 5 5
then type ~reroll confirm
It has limits on how many you can put of course (for legit-sake) and if you have traits that alter your SPECIAL, enter it in without them.
So basically if you have small frame, put your agi 1 lower than you want it to be and it will come out how you want it.