I'd say you don't need a blanket because they can over heat I know it's not the answer to your question but if you got one get a light one also your local tack store could probably help and you could bring him in a trailer there and they could measure him maybe x)
Very helpful! Thanks for all the responses!
66 would be an ideal fit but expect to go even bigger. possible a 67 but it may be a bit big
I would suggest getting her a 66 so she has time to grow into it a little bit
I would say maybe a size bigger than 66 because if she's gonna gain muscle and grow some more
Try getting her a 66 it wont be snug but it wont be hige either, and she can grow into it.