I guess I could go with Small, but they're awfully long for a rattler (and I used to wrangle them!).
I'd rather stick with Medium as well, since the Stormwrack eel is actually smaller and still Medium.
Well according to a my notes from
The Guinness Book of Animal Records the World's heaviest venomous snake was a 2.36m and 15kg (7' 9" & 34 lb) Eastern Diamondback (
Crotalus adamanteus).
The Eastern Diamondback averages 1.52-1.83m and 5.5-6.8kg (12-15 lb and 5-6' long), about the size of a Weed Eel. 12-15 pounds is definitely Small size territory. Furthermore, rattlesnakes can be fairly chubby for snakes, the Weed Eel could be slimmer.
Of course, we should be comparing Eels, not Snakes. A
giant moray (
Gymnothorax javanicus) can approach 10 ft. long and 60 to 80 pounds depending on who you believe. Scale that down to 6 ft. and it works out around 13-17.25 pounds.
Let's see how other serpent/worm shaped creatures from the SRD scale:
Purple Worm (Gargantuan & 80 ft.) => 10 ft. Medium, 5 ft. Small
Frost Worm (Huge & 40 ft.) => 10 ft. Medium, 5 ft. Small
Naga (Large & 10-20 ft.) => 5-10 ft. Medium, 2.5-5 ft. Small
If the Weed Eel is shaped like the Worms, 6 ft. is closer to Small than Medium, if it's shaped like a Naga it's easily in Medium.
Of the Worm/Naga models I definitely prefer the Small Worm. I've always wondered why Nagas are so chubby in 3rd edition.
The 6 to 8 inch diameter tunnels is what clinches it for me. If you really want to make them Medium I'd fuss about giving them a "squeeze through tight places" Special Quality.