Is there anything anyone has to say about the identity of its producer?
My best guess is Eurasian Jay, but I have no way of knowing - and only knowledge is power.
Regarding its size, I didn't have a measuring device with me and it was starting to rain, so I didn't have the time or rather didn't feel I had the time to place anything of know size beside it. You know, these things you always chance upon when turning your pockets inside out (credit cards, 100 Euro bills, diamond rings etc.).
Suffice to say - I hope - that my hand is normal sized for a human hand, fingers could be longer but as I don't attempt playing the bass guitar anymore, I am rather fine with the shape they are. So I just measured my hand and would guess the length of the egg was around 35 to 40 mm.
In case that might help.
Cheers in advance and bird on.