Choosing Rondeau

Hello, I'm thinking of getting a rondeau but I have a few questions:
1) What size would be ideal for 2 adults and 2 small kids (maybe more kids in the future lol)? Occasionally I do cook for more guests. I was thinking of 30cm (5.8qts I think?) or would that be too big?

2) Is there a significant difference between a stainless steel and copper rondeau (besides the price of course)? (I'm looking to get a Mauviel if that matters)

3) For copper, is there a big difference between 1.5mm and 2.5mm (besides the price again haha)? I've seen some discussions on this but was just wondering if it's a personal preference or are there limitations on what kind the 1.5mm could cook or something.

I'm not thaaat fussy, I cook almost everyday, like to try different kinds of cuisines.

Thank you!