The constrictor snake presented here is a relatively small one. You can create stats for a larger maneater like an anaconda by applying the advanced and giant simple templates, or by advancing the stats above to a 7 HD Large snake (CR 5), or even a 14 HD Huge snake (CR 10).....
Venomous snakes are generally far more aggressive than constrictor snakes, and even larger variants do exist: you can create stats for a king cobra, for example, by applying the advanced and giant simple templates to the stats given above.(Those are the stats of a medium sized snake)
RAW you can't wildshape or use beast shapes to turn into a templated version of a creature.On the other hand the description of the snakes say clearly that in nature larger versions that those depicted in the bestiary exist. Those versions aren't printed in the rules simply to save space.
So how do you think it should work:
- a druid or someone using beastshape II and above can't turn into a large snake unless he can find a printed description somewhere;
- a druid or someone using beastshape II can turn into a large snake as they exist in nature.
For the RL argument:
The king cobra (Ophiophagus hannah) is an elapid found predominantly in forests from India through Southeast Asia. This species is the world's longest venomous snake, with a length up to 18.5 to 18.8 ft (5.6 to 5.7 m).