Recalling Neurosurgeon conversation, below 5mm is considered small. Between 5 & 7 mm is medium and over 7 is large

I was diagnosed a year ago with a "posterior communicating artery" aneurysm. Unfortunately, it was found during an MRI check up for a meningioma I had removed via brain surgery. I wasn't in a space to deal with my new diagnosis, the aneurysm, so I did nothing for 6 months. I happened upon the most informative site regarding aneurysms that answered all my questions. Dr. Omar Chouddhri is a neurosurgeon at the University of Pennsylvania. I found a 35 minute presentation given by him on You Tube.com by typing in "Omar Chouddhri aneurysm video" in the search bar. He explained everything in a way a non-medical person could understand. I had already researched many websites with information on aneurysms through Mayo Clinic , Cleveland Clinic, Stanford, etc. but after watching that video I made my decision that I needed to follow up and have it coiled. So I took a quick mini-vacation to Hawaii and then came home and got serious about conferring with a neurosurgeon who specializes in aneurysm repairs. I had the procedure a week before Christmas. Not a big deal. And to answer your question, my understanding is that generally, any aneurysm over 5 mm needs to have an intervention of some kind. Once again, as with meningiomas of the brain, one's age, the location of the aneurysm, and it's size all play into the decision of how to deal with it. Hope that helps.