Just had an ultrasound and my ovaries are different sizes. Is this normal?

Usually. Variation of several millimeters is not uncommon and sometimes a technical issue on the part of the person performing the scan. If the ovaries are otherwise normal in appearance there is no reason for concern. If one ovary is twice the size of the other it may be useful to perform a followup ultrasound in a few weeks or, in some cases, evaluate with MRI depending on the clinical concern.