This!
OP, you are quite blessed (and probably have lovely feel/body awareness, plus some good horsemanship) to have a young horse who is uncomplicated in his mouth. I try to have/make these kinds of horses. It makes my life easier and cheaper. IME, that extra 1/4 inch doesn’t matter too much, so if you have a bit that he says he likes, don’t worry about the size.
I have liked, especially and recently, for dressage horses, Stubben’s anatomical bit. It’s about $80 and I think Smartpak has it in a 5". But I tend to like an extra 1/4" in a loose ring and in an anatomical bit.
https://www.smartpakequine.com/pt/stubben-anatomic-loose-ring-snaffle-bit-15208?utm_source=google&utm_medium=cpc&utm_content=NB_Shopping_Tack_DSK_3P&utm_campaign=NB_Tack+>+English_Bits_DSK__3P&gclid=CjwKCAjwoNuGBhA8EiwAFxomA9ee6Yqd-cuWY0l4zG6dMQMDXkMhPJ9_v5uqG8kkJtt6XZvOdSDCfxoCLVUQAvD_BwE&gclsrc=aw.ds
I like this bit because it’s not too fat for a small mouth. The curve makes it comfortable for the horse to “pack” or carry himself in his mouth. And I like it better than, say, the Neueschule Verbinden bit because’s it’s a tad heavier. I think that gives the mouthpiece some stability and predictability when it’s lying on their tongue. But I’m just speculating here, and pretty wildly so, LOL.
And I hate my Verbinden-- it’s too light for my light breeds-- so if any of y’all want to buy my 5.25" that has been in a mare’s mouth just once for less than I paid, hit me up.
