It seems on the picture with wardens and Skavens that the basis are the same for both units, so I'd say 25mm... But I'm not sure, maybe should I wear glasses ^^
I thaught it was once per battle for every model with this aptitude. But you may have right : it's only once per battle, no matter how many models have the aptitude.
Taking the battalion with 2 TLA also means 3 awakened Wyldwoods without any chance to dissipate for the opponent. I don't know if that worth 600 points, but I think it's something to keep in mind. And 3 wyldwoods will give potentially more efficiency to TLA spell. Last, even if TLA is maybe a bit overcosted (2 spells could have been great), it's really resistant and can be a really good support.
(Sorry for my English by the way XD)
I think we'll have at least a third endless spell, like every faction (except Gloomspite which have 4 ^^), and maybe a terrain piece if the tree isn't one.