Can an ovary change in size from month to month?

Unlikely. I'm not sure why you are asking this question, it seems like you had 2 imaging modalities a month apart that showed different measurements. Change in size CAN be normal especially that the ovary is 3 dimensional and the measurements we do are 2 dimensional so there might be a slight difference that is OK. That being said, it could also be abnormal, so keep an eye on it.