possible wayIt can't really be done during a necropsy. Firstly one would need a dead chicken with a crop full to capacity. Lots of changes happen to the body shortly after death.
With a set of calipers to start with. Later I filled a balloon with a quantity of feed (dry crumble) and felt a crop and then the balloon; adding or subtracting until the balloon and the crop felt the same. It's not exact but it's surprisngly close.
Take a hungry chicken
measure a bunch of feed, or weigh chicken
let said chicken eat until full
measure feed left, or weigh chicken