How old exactly?
I would say stick to the rule of no longer then the space between the eyes, as everyone else has stated. If you jackson won't eat, i would try perhaps feeding a different feeder.
My "Ace in the hole" Or rather "Aces" as far as getting stubborn chams to eat are:
Flies, ( Blue Bottle Flies are my favorite)
Moths,( Keep them overnight to ensure no pesticides came in contact)
Phoenix Worms
these are temptations none of my chams have been able to resist.
Ps, Welcome onto the Forums! Always a pleasure having new faces on here.