Sorry, not helpful but I'd go without although I might change my mind if I move up North!
If you're buying something like a Rhino rug, I'd go for 6 inches as it will easily drape down over the quarters but the chest will still fit. Any other type of rug with a neck, I'd go for 3 inches purely because it won't fit as well at the neck which could cause problems. Also bear in mind, neither are likely to last sizewise for longer than this one winter as by next winter horse will have outgrown it easily.
Maisie is now wearing a 5.6 sheet with a knot in each surcingle, the only bit that is far too big. It does drape over her tail but room for winter growth. A 5.3 i have that was her mothers is a little short over her bum but to go to 5.9 would bury her. I have sheets ready for ebay that both the babies wore from 6months old with no problems. However, i didn't rug them until they were weaned, partly because Bea wouldn't let her baby anywhere near to drink if she had a rug on (poor baby)!!
I used Amigo's on mine at that age, the pony sizes are small enough up front so 3 or 6 inches at the back isn't a problem.
Other than that, other makes and bigger sizes, you can get away with the 6" if you put a few stitches in the front and raise/tighten the shoulders then let the stitches out when they grow more. Plaiting thread is V good for that!