For me, it would depend on how close to too small the 5" was. If it's one of those "Well, he is about to outgrow his bit, I better think of getting a new one soon", then I would stick with a 5.5" for the time being. However, if it's more like "Wow, this bit is definitely too small, it's tight on the corners of his mouth", then I would probably go with the 6". The good thing about snaffles is that they aren't terribly expensive in the grand scheme of things so if you have to get a bigger one here in a few months, it won't be such a big deal.
As for your friend that keep insisting you bit him up...keep ignoring her. He's your horse and what you decide to do with him is completely between you and you're trainer (who sounds like a fantastical trainer, BTW).
Oh, ETA. I don't know what your eventual goals are with Aires, whether you eventually want to move up to a curb bit or if you plan to stick with a snaffle, but I thought I would let you know that I plan to get a bit for Rafe from these folks
sometime next spring. Since nobody makes bits in the style that I like in draft size, I'll have to go custom and these people have the most competitive price and their stuff looks like amazing quality.
It's more of a "Hmmmm...we need to think about getting a bigger bit sooner rather than later" than a "OMG! We need a new bit...STAT!"
I love my trainer. She is amazing and a good friend, too. She has stuck by me and made me feel really good after everything that happened a couple of weeks ago. She gets what I want to do with Aires (in the long-term and short-term) and is totally on-board with it and with training him toward those goals.
As for my long-term goals...they don't involve much of anything Western, truth be told. lol We might dabble in WP and even try out team sorting at some point, but nothing serious. The main thing I want to do is eventing. Aires has stamina for DAYS (one day when the BO was working with him, he had to lunge him more than usual to get him to listen and after 30 minutes of working him hard, the BO was sweating more and breathing harder than Aires was) and has such beautiful, ground-eating movement. Our trainer thinks he'd excel at it, as well.
I would measure his mouth and go with whatever size that is. Its best, especially for the greenies, to get a bit that fits as perfect as possible. Also, I know there are some websites that will let you "rent" a bit for a week and send it back if it doesnt fit or he doesnt like it.
As a fellow big horse owner, I know how hard it is to find big bits
. However, I found this site if you need to order. Horse bits, Neue Schule bits, Myler bits, Sprenger bits, Abbey bits, Reinsman bits, bitting accessories and advice - HorseBitBank.com
I've already found some decently-priced bits on eBay, so we're good there. In fact, DraftXDressage had (still has, hopefully?) a 5.5" D-ring french link for sale, so I'm going to contact her and see if she still has it. The price she has it at is comparable to eBay's prices and I'd rather buy from someone on here, to be honest.
I would do the 5.5'' myself....Just because, moving up too much could be just as bad if his mouth is too small for the 6''.
As for the friend, I would probably end up making a snarky remark...But that wouldn't be the mature way to handle it >.> Just ignore her and show her how good Aires is later on
The thing about my friend is that her heart really is in the right place. I love her to death and she really just wants Aires and I to succeed. She's already impressed with Aires' temperament (didn't believe me that he was a two-year-old when she met him the first time because of how calm he is) and how well-trained he is on the ground (hand-trotted him at one point while she was there and didn't even barely have to coax him into it). A snarky remark would really hurt her feelings and that's the last thing I want to do.