A GCC would need a petite (xxs)
Actually my Green Cheek wears a size "Extra-Small" Aviator Harness, not an "Extra-Extra Small"; I have only one size double extra-small Aviator Harness, and the only birds I have that it fits are my Budgies (would probably also fit a Parrotlet), but it is way too small for Bowie, my Green Cheek...
I have several Aviator Harnesses, all are size single "Extra-Small", and Bowie (Green Cheek Conure), Duff (Cockatiel), Lita (Quaker Parrot), and up until he put on a bit of weight around his mid-section Kane (Senegal Parrot) all wear the same size, an Extra-Small; the Budgies, both American and English, wear the double "Extra-Extra Small"...I actually just ordered a size "Small" for Kane because the Extra-Small was getting tight on him...But there is no way that an "Extra-Extra Small" would ever fit Bowie (he's a male Yellow-Sided Green Cheek who ways right around 78-85 grams at any given time, so he's not a huge Greeen Cheek or anything)...The Aviator Harnesses have quite a bit of adjustment to them, so the Extra-Small actually gives them all a nice bit of adjustment for weight-changes and among my different species that make-up my flock...But I wouldn't order an Extra-Extra Small for any Green Cheek Conure, Crimson Bellied Conure, Black-Capped Conure, etc., because even at it's full adjustment to it's largest size, an Extra-Exatra Small wouldn't even go on Bowie, let alone be comfortable or allow him to move...
I'll also just put in my vote for the Aviator Harness to be the best parrot Harness/Leash available (BY-FAR!). I have seen a lot of different bird harnesses over the years (most of them I don't even know the brand or model name of, as they come in to the Rescue with birds that are surrendered in their belongings that their owners send with them), and none of them are as easy to get on the bird, tighten or loosen while already on the bird, or are seemingly as comfortable for the bird while they're wearing them...The Aviator Harnesses are also the SAFEST harnesses out there in their design, as there is no way that they could possibly strangle or choke a bird, and most-importantly they won't ever cause your bird to be "Yanked-Backwards", because they are actually made to allow your bird to be able to fly while wearing them and being attached to the end of the leash. I am able to run alongside of both Bowie and Kane when they are wearing their Aviator Harnesses and have them fly right along with me (Duff and Lita don't get it, we've tried, lol)..But what's great and most-important is that they will simply glide safely and calmly to the ground whenever they stop flying, so they never get "yanked", choked, hung, etc. And of course they are attached to them around their entire body, as opposed to having the point of attachement be one particular body-part or area like their neck, leg, foot, wings, etc., which always cause injury and even death. So the Aviator Harness gets my full vote!
). Cairo and I go everywhere with his Aviator harness - walks at least every other day, some Cairo-friendly restaurants, friends outings, anything and everything! I am more than happy to walk through all the details of harness training (admittedly, Cairo picked up on it abnormally fast). And a lot of people have active threads about their ongoing harness training - go and poke around and pester them as well!! It's the best thing for Cairo, who used to be free-flight trained. He's getting so comfortable with being outside. And he's a much happier bird with the time spent outside and in new/interesting places.
