Brown Betty is a classic style as much as anything else, so outside of some seriously pedantic, likely highly debatable historical points, any of quality manufacture is as authentic as any other. The size thing....somewhere along the line a decision was made that a "cup" of hot beverage, be it tea or coffee was six ounces which likely as not had to do with the size of cups in fancy china services, since the coffee cups I'm used to in the US and recall from living in the UK were generally 8-10 ounces for the regular size and what is the limit for the "large" mugs. The
24 ounce was the most common I recall seeing in shops in England if that's important to you. What I would do is consider how you will be using it. If you never expect to serve company and will only be taking a couple cups before it has a chance to go cold even with a tea-cosey, a 2-3 cupper will likely be sufficient. If a guest/companion/whatever is in the mix then adapt accordingly and if it's a bit large for everyday needs and only one pot is in your budget at the moment, then there's nothing that says you have to make a whole pot every time you use it.
Tea-pots are marvelous little things. They only require care when you use them and they can be a joy to look at even when you don't. One of them is likely sufficient, but another one can be a welcome addition....and maybe just one more.... and even though I really don't need any more, there was this one I happened to see at Whole Foods today.....
-Doc