Hmm, tricky. The SRD Wyvern has identical dimensions to the Bloodstinger, which argues for it being the lower end of Large, but on the other hand the Bloodstinger's bite and sting damage are pretty close to a Wyvern that's been downscaled one category. (Bite 2d8 -> 1d10, Sting 1d6 -> 1d4)
I'll go for Medium, since the 2E Wyvern is about twice as long as the 2E Bloodstinger (25' vs 15'). We could halve the length of the Bloodstinger to 7 to 8 feet, including a four foot tail, plus a 10 foot wingspan. It should weigh about 250 pounds.
I'd use the stats of the Wyvern, but drop its Int to 2 (animal intelligence) and cut its Strength by 4 (it should be weaker than a Wyvern but I don't think it deserves the whole +/- 8 Str advancement between Medium and Large):
Str 15, Dex 12, Con 15, Int 6, Wis 12, Cha 9.
Natural Armour would probably be +6, giving it AC17 with its Dex.
As for its poison, I suspect the intent was to duplicate a slow spell's effect:
An affected creature moves and attacks at a drastically slowed rate. A slowed creature can take only a single move action or standard action each turn, but not both (nor may it take full-round actions). Additionally, it takes a -1 penalty on attack rolls, AC, and Reflex saves. A slowed creature moves at half its normal speed (round down to the next 5-foot increment), which affects the creature’s jumping distance as normal for decreased speed.
A minute of that followed by Con damage is pretty nasty.
If you fancy a less cruel option, what about the initial effect being similar to fatigued, exhausted or sickened?
Say, the victim can not run or charge and suffers a -1 or -2 penalty on attacks, Reflex saves, Dexterity checks and Dex-based skills for the poison's duration?