Feeding jars and bowls are usually used to prevent axies ingesting too much sand when feeding, so that kinds point to the axies being over 5". until then a bare bottom is recommended, and axies can quite happily root around on that for food.
I never had any luck getting mine to use a jar - they all seemed nervous about an enclosed space, and even now prefer tunnel hides over caves.
A bowl will probably not be any use either, the food tends to get scattered as the axies feed, so there's not much point in using one.