What size would you recommend ?
Whatever size matters. I ended up buying a 512 GB with 512gb micro SD. I have like three games that take up 100GB (Elden Ring, Witcher 3, and some other game I forget), and like fifty indie games.
What's the battery like when running games or watching movies ?
Depends. It's like a laptop, and depends what you're doing.
I play Elden Ring while it's plugged into my dock + wall outlet, so it's on the big TV. If I played it while portable, it'll last like 1.5 hours.
Where I can play Vampire Survivors or many 2D metroidvanias for like 4 hours.
Any gotcha moments when running a deck?
Be careful because if you're like me, you might no longer build gaming PCs ever again.
I say that and honestly, most graphic intensive games don't look great on Steam Deck. Like Cyberpunk looks great on a mid-level gaming PC where it looks like Lowres muddy blurs on Steam Deck. But if you never knew how it originally looked, you might not complain.