Does it matter what size you get? There seem to be loads of different sizes available but wasn't sure if it actually makes a difference provided the horse can get a clear look at themself in it.
If your horse is fixated on mirrors - mine is - you can get the smaller ones which are really designed for trailers. But I think they prefer it if they can see a fair amount of themselves. We have a small one in the trailer for him and a larger one in his stable. You should see him at an indoor school which huge schooling mirrors. He can admire himself for hours!