Fireworms or beardered fireworms can grow between 1 inches to 3.93 inches long.
However sometimes a fireworm can grow as big as 13 inches which is 1 foot and 1 inch long although most are smaller than that.
I have seen fireworms that were at least 12 inches long though so it's possible.
Most fireworms you'll see will be around 1 inch to 3 inches long though.
You should never handle a fireworm because when you pick them up they can then use their bristles to stick your skin which can then introduce poisonous venom into your body.
That poisonous venom that a bearded fireworm has can be deadly soon if you do not get medical attention soon.