Aviator size/how to train?

I want to say medium? For my Jardine's parrot? but some places say small? So I'm confused haha..

Also, how would I go about training her to wear it?

I followed the weight recommendation for my eclectus, but regretted it and ended up getting a larger size. My bird was about 370grams, and the weight recommendation for the 'med' was 425, which sounded like a big difference so I got the small size. The small did technically fit and I could get it on him, but the head hole in particularly was so small it was hard to do. I got the bigger size and it is a LOT easier. They are adjustable so even if it's a little large I imagine it would fit anyway.

The harness comes with an instruction DVD on how to train. I recommend you follow it. I was like "Oh he doesn't mind putting it on!" when he was a baby so I didn't bother following the recommendations. When they are older, they won't let you put it on as easy, I guarantee

My bird makes a bit of a fuss, but he doesn't bite when I put it on now. (Two people putting it on> One holding him, me putting on the harness) does make it easier and the harness goes on 'neater' so it doesn't hurt him when his feathers get moved around.

My bird LOVES going for walks. He enjoys the scenery and he is a lot happier immediately after a walk. So it's nice to do that for them.