Tervigon Size?

So, I'm currently working on a 'Nid army, and I've come to a bit of a quandary. Most of the Tervigons I've seen have been built off of the Carnifex and placed on that size base. However, the statline leads me to believe that it should actually be based off of the Trygon, at least with regards to size. What do you guys think? The larger base could help with some of the 6" abilities and even with Synapse and Shadow in the Warp.

I think I had heard, somewhere, that the tervigon was supposed to be a kitbash between the 'fex and the trygon. Sadly, it's GW so there isn't an official answer.

I haven't seen anything that officially says you have to mount it on a trygon base. IMO, I prefer the carnifex base mainly because of model positioning and movement phase logistics. If it was on a bigger base, I could gain more spawn radius but it would be around the sides and back (front would be the same no matter that base). Not seeing the benefits. Also, what else am I supposed to do with my shelved carnifexes? :)