Why other number crossed for my car?

if you hover over the little code it should tell you what variant its for, the one next to the quantity column

What is the best large impeller or small impeller?

well i would say logic says the bigger will move more water, but i'm no fluid dynamics expert lol.

The other side is more mass of water will put more stress on the bearing resulting shortening the lifetime. I dont know exactly.

It's 6 and 2 3s really. Just get a decent named brand one and should be fine mate. I'd not overthink it too much