I'm building a 1/8 hydro for just fun running around my lake, so I don't have to fit any specific motor requirements to fit any rules. Since that opens up pretty much any set-up that I could ever want, I started shopping around, and getting confused. Even thought I won't be racing others, I still want a fast boat.

KV ratings vs motor diameter vs motor length - In real world applications, what would be the difference between a 56mm x 92mm (like a TP 5670) running 850kv, vs a 40mm x 102mm (like a NEU 1527) running 850kv? In theory, they would turn a prop at the same RPM, right? But I'm guessing the bigger motor would have more torque? So it could run a bigger prop? I get the bigger motor would be heavier, but would it be able to push a boat faster? Plus, the TP5670 is way less expensive than the NEU 1527.

Motor KV ratings - I've read that it is good to think of them more like a transmission where a lower kv rating is like a lower gear that can turn a bigger prop slower instead of a higher kv rating can spin a smaller prop faster. I also see that a sport boat should run around 25k rpm and a racing boat around 30k rpm. I calculate a kv rating motor of 950kv on 8s or 10s would spin at around those goals. Since I'm not stuck with required motors, would this be a good kv rating for not being limited by rules? I'm sure there are lots of variables that I don't know about. I'm sure prop size is the big one that I don't completely understand.

Sorry for the technical questions, but I'd really like to understand this instead of just blindly ordering a motor that looks good.