Your manual is correct 2 and 1/4 inches + or - 1/4 inch. I found this old review in MAN.

[link]http://www.retail-leaders.com/reviews/123662man.htm[/link]

Try and get a flat pack battery up along the left side of the tank. A Hayes 4oz slimline tank may help. You probably won't get much bigger than that in there. And you may not want more then 4oz. You'll be pretty rung-out before that 4oz goes dry. I used a too small 270mah nicad with the stock tank, I think. But I had to check the charge before every flight. Now with NIMH batteries you should be able to find around a 500mah to fit.