Teapots...what is up with these sizes?

I am looking to buy a nice cast iron teapot. The kind that is coated with enamel to prevent rusting on the inside.
I look around and see the sizes! A 20 oz pot is what I call a single setting serving. It will make 2 nice mugs of tea and although this is suppose to be for 2 people, it would be for one around here!
Think about the size of 20 oz. That is one of today's vending machine sized Pepsi bottles.

The next size up is 40 oz, that is a little too big and then there is the 85 oz. WAY too big.

So I guess if I do purchase a iron pot, which is what I want, not a kettle. I will have to go with the 20 oz pot and then fill it up for the next "sitting". The fortunate thing about cast iron is once you proof the pot, it will have enough residual heat to keep the tea warm for quite a bit.

I have several pots, stoneware, china, even a metal one that I don't care to use but keep because of its beauty. I have infuser pots, french presses, etc. etc.

The one I use for when I come into my office here (when I get caught up in my computer and forget I have a pot of hot tea next to me!) isn't even a teapot. It is a thermal dairy carafe.
If you have ever been in Starbuck's you have seen these carafes, they hold the cream, half & half and milk near the sugars in that store. I bought mine in a restaurant supply store. It holds a quart. Perfect for a morning or afternoon at the computer. They are stainless steel and if you proof them well, they will stay hot for hours.

Well, I am off to look for my pot on ebay. Wish me luck!

Brightest Tea Blessings!