For the kale, I've done it 2 ways (both ways earned me death stares... like if I wear the color red lol). I cut it up in small pieces and put in the cup (I have added feeders to the cup but she is a pro at picking around the bits of kale). I have also taken a couple leaves and, using a chip clip, attached the leaves to a vine. They dry out fast.
The melon and apple I cut up in smallish pieces. I cut up the apple slightly smaller because it is a harder-fleshed fruit compared to the melon. I cut up the melon a little bigger which helps her eat them. I wish I could easily give you dimensions! but use your best judgement/common sense. If the feeder insects are soft-bodied, the can be a little larger then the standard advice which is nothing larger than the space between their eyes. Hope that makes sense!