Follicle measurements

Hi all, so I had an my first ultrasound on CD14 to check follicle number and size etc and they said they measured between 2mm and 7mm on right and 2mm and 8mm on left ovary. I had another ultrasound today (CD18) at a different place and they said my biggest follicle measured 10mm by 8mm? I am a little confused as to how big this is, I mean do I need to work out the diameter? If so how do I do this. Can someone please explain. Thanks in advance!