I don't think any 3DS game has used an 8GB cart yet. Resident Evil: Revelations uses the most memory so far which is on a 4GB card IIRC. Most Nintendo games have been either 1GB or 2GB so don't expect Game Freak to go overboard.
I'm thinking 4GB at most. Fire Emblem Awakening is (I think) a 2GB game and it is ~11,000 blocks. Pokedex 3D Pro is ~2500. Since this game will (probably) use all those models, just with cel-shading, it will be at least 2500 blocks, most likely a lot more because it's an actual game as compared to Pokedex 3D Pro.
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