"Thomas G. Marshall" wrote

Ok, our 40 gal gas water heater is failing. About 10 years old, which I
hear is ok.

Should be 15 but if it's hard worked, perhaps 10 is ok.

But every time I talk to someone about their 40 gal water heater, they all
complain that they run out of hot water (comparable size homes and kids).

40G is ok. Not optimal for long showers if there are several and need to
take them within 20 mins of each other (night time shower with several
kids).

But we were always worried about running the wash or dishwasher before
showering, or two long showers would result in the 2nd losing water.
Complete with those water saving disks etc.

Yup. If you can, go 50/60G. More is overkill.

1. Is 80 gallons overkill? Perhaps I'm reacting viscerally.

Yes though it will let all 4 shower within minutes of each other.

2. Am I going to notice a large gas usage increase?

Not with a mere 10-20G difference

3. Are there rules in place limiting the water tank sizes?

Has more to do with construction of your place. If it has to fit in a
cabinet for example, that cabinet may not be sized safely to hold a larger
unit. There may be distance from the walls for example involved.

4. Are there any particular brands to look for or stay away from?

Sorry, cant answer. If it helps though I have a 40G GE 'Smart Saver' gas
hot water heater. I do have to be a bit careful to not take a shower or
bath right after running the dishwasher or another's shower. 20 mins seems
to work for the wait. It's 14 years old and i seem to recall the wait 10
years ago was about 10 mins so we may be getting near replacement time.
IT's in the garage so there is no space problem if we get a bigger one.