i dont have any exp with j. jacksonii, but i would imagine xanths are similar, xanths seem to be more concerned with the way the cage is planted and setup rather than overall size, and having the top of the cage above eye level seems to make them more comfortable, even still, i would consider the smaller size cage mentioned, suitable (for a solo adult female) as long as you didnt go any smaller and the top of the cage/ basking spot is above eye level. i would think that cup feeding would better for the long run, if the animal is healthy and eating properly, acclimating it to cup feeding should be a minor issue, if its not eating regularly, then feces would be the best indicater of health. if feces is normal, then things are probably ok , if feces is often subnormal, then you may have bigger issues than cage size or feeding style. with all due respect, this is where the help form comes in handy, with little information about the animal or the way it is kept (or condition of the feces) it is hard to ascertain what issues might be in play