I don't know about JGs specifically since they apparently range is size from "a bit larger than a standard LF" to "No, it isn't a turkey"
But 14x14 is plenty big for any LF, in fact I think 12x12 is pretty common. My bigger Black Australorp is probably as big as a smaller BJG (clearly larger than my other hens) and she fits in a 12x12. In fact she and 7 chicks fit in there from 2 weeks to almost 4 weeks of age.
What if you make a box 14 inches deep and "x" inches long with movable dividers so if that 14x14 seems a bit of a squeeze for yours, you can reconfigure it? And do you have or plan to have other breeds? If so and you have to move a divider, they will be fine with a smaller box. Heck, since they sometimes have favorites and are stubborn about it, you might have 2 in the same box anyway.
Edited by bruceha2000 - 10/22/15 at 7:19am