How many burners are you going to use, and what style?
Do you plan on forge welding?
If you use the Reil style burners or the Zoeller style burners you'll need 1 burner per 350 cubic inches of volume.
If you use 2 inches of insulation a 14 inch length will have a volume of 704 cubic inches and would require 2 burners.
I prefer to keep to keep the firebox a small as possible, no sense in heating a large unused space. I also prefer to keep the ratio close to 250 cubic inches per burner, it save a ton on fuel in the long run.
Building your own forge is fine, problem is, there's a ton of info online about forge construction, and a whole lot of it is just plain wrong. It seem if someone is able to build a forge and actually make a piece of steel orange with it, they are suddenly an expert and just must create a webpage to share their wealth of knowledge... be careful!
Use a good insulation. For a cylinder, try 2 or 3 layers of 1 inch ceramic fiber coated with colloidal silica hardener (sprays on like water...), a coating of ITC 100 wouldn't hurt either. Use a good floor material, some dense ceramic refractory splits (4 1/2" x 9" x 1 1/4") work well, are inexpensive, and easy to replace when the flux buildup get to be too much. I don't like the idea of ceramic fiber in a welding forge because the flux will literally melt it away on contact, but if you are using a round design, there isn't many options. You could use a cast refractory, but there goes your efficiency... Cast refractory will suck the heat right out of your forge and you'll struggle trying to maintain welding heat, if you even can get that hot. I see virtually no difference between a round or square forge as far as even heating goes (and I have built HUNDREDS of forges), and a square body gives you a ton of options as far as lining material goes.