Reposting in better section: What's the average size of a normal crop?

Hi all!
A few days ago I posted a thread about my chicken Petunia and this funny head-bobbing thing she does. Many of you said to check her crop; that it looks like something a chicken does when she's trying to move stuff through her crop/ or that she might be trying to "grind" food in her crop. I had thought it was just a silly behavioral thing (since she's always been a bit of a peculiar girl), and not something related to a physical ailment...so you all got me wondering.
So I just went out and grabbed her and felt/massaged her crop. The problem is...I don't know what it's supposed to feel like, so I really have no basis for comparison!
Here's what I felt:
1) a crop about the size of a racquetball (smaller than a tennis ball, bigger than a golf ball),
2) it was semi-firm, but pliable (it didn't feel hard in any way, just felt like a toned muscle rather than a floppy, squishy sac),
3) Petunia didn't fight me or appear in any discomfort as I did it,
4) I could not feel any lumps or hard objects,
5) Nothing "came up" as I was massaging (no fluids or solid pieces),
6) I gently opened her mouth and did not smell anything sour or malodorous (had read about "sour crop" in my research).

So, my questions are:
What's the normal size of an empty crop?
What is the average size of a crop when the chicken has been doing her "normal" thing (since I checked her crop in the middle of the afternoon)? (I'll check it again first thing in the morning...)
What should a normal crop feel like?

Is all this worrying for nothing? She's acting pretty normal, other than she hasn't been laying any eggs for a bit and the silly head thing she's doing. She seems to be eating adequately (she's not skinny/hasn't lost any weight that I can tell) She is active; loves her dust baths, she's not lethargic, her feathers look shiny and nice, eyes are clear and alert and she's not being picked on by anybody...what do you think?