Hermie at full scale is 20mm (3/4") in diameter, and as long as you like (48 reg. segments will make him about 65cm overall- over 2 feet long)! You will also need some .5mm elastic bead-cord, and a small hook or piece of wire to put him/her together. Print him in color or paint her like I did.
Some quick earthworm facts (that I learned while creating this):

-In one acre of land, there can be more than a million earthworms.
-The largest earthworm ever found was in South Africa and measured 22 feet from its nose to the tip of its tail.
-Earthworms have the ability to replace or replicate lost segments. This ability varies greatly depending on the species of worm you have, the amount of damage to the worm and where it is cut.
-Baby worms are not born. They hatch from cocoons smaller than a grain of rice.
-Even though worms don’t have eyes, they can sense light, especially at their anterior (front end). They move away from light and will become paralyzed if exposed to light for too long (approximately one hour).
-If a worm’s skin dries out, it will die (it breathes through it's skin).
-Worms are hermaphrodites. Each worm has both male and female organs.
-Worms can eat their weight each day.
-Worms have 5 "hearts".

Assembly:

1) Tie .5mm elastic cord around the eye on the bottom of the head.
2) Thread pieces onto cord (except for the butt). Put about 4-5 regular segments between the head and the clitellum (the puffy-bit).
3) Stretch cord firmly to determine length, and tie end to hook.
(hook should be kinda tight against body at this point, holding pieces together firmly with a bit of stretch left)
4) Attach hook to butt (see photo).