What size wire should feed a dinning room?

I would use #12.

Load determines size. 12 will handle 20 amps which is more than enough for most homes, mine is #14 handling 15 amps just fine

Code requires a 20 amp circuit with #12 wire for receptacles only in a dining room.

Down here, we can't use #14, I have used #12 for so long, and have seen #14 and a 15 Amp Breaker not be enough for a space heater, or much expansion, to not use it even if allowed, MAYBE lighting.
Most of the Cost is Labor and the difference in cost between the 2 is not worth "downgrading".
I would rather have extra power available and not need it then to be lacking. My Opinion.