There should be an option there in a drop down box.
Long legs are basically for Gibsons and other guitars with a noticeable neck angle and/or pickguard mounted pickups. Sometimes the cavity is too shallow for long legs.
EDIT: I just checked and that's really weird. There is no option for leg length on Juggs. I checked Cold Sweat and the option is still there for that model. Maybe use the messaging function on the web store to ask directly. The option is available on all of the other ones I checked - Painkiller, Aftermath, Warpig - except for the Black Hawk