Jenny -
Hi there and welcome to ADBB
I would say that the 1/2 an avocado that you're allowed, is more than likely based on an average sized one. I have not eaten one as of yet lol.. I'm sure that you will get some other replies from fellow ADBBers, that may have more knowledge on this subject. I'm suspecting though, that you should go with your intuition on this one.
I wish you great success on your new journey.
Sassy