Do i need to stop using nasonex (mometasone) & anti-histamines prior to radio frequency turbinate reduction? will using them give a wrong indication of the size?

Probably not: I presume that your are undergoing reduction because the turbinate hypertrophy has not responded to medications. Thus whether you use the medication or not is not important if your nasal congestion has responded well to medications, then i would caution against having a turbinate reduction which will likely provide only temporary relief. You should talk to the surgeon re. His preference however.