Winter Blanket for Weanling

Its October now, and I live in Ontario, so that means that we will be getting snow within a month and a half.
Sonya has her blanket, but I need one for Rumour.
I know how to measure to see what her measurements are, but I don't know whether I should get it a size bigger, as she will be wearing it from November- February at least, and growing. She will be inside when the weather is terrible, but will probably be out in -20 degree weather most of the time.
So what size, and should I get a moderate, or heavy? The barn she is in can get a little cold, so she will still have her blankie on inside.
Thanks!

Hi. I would get a size bigger. Saves from having to worry about it later. I would try and get a 1200 denier waterproof. Be careful of it being to warm for her as she could sweat and get the chills. I would wait to put it on her until its actually very cold so she can build up her coat. I was worried about my yearling filly this year but its a tough call because once it is on you really can't take it off. Maybe only on decent days where it isn't raining or snowing. My horses are out 24/7 so I don't know what I am going to do yet. You could go with a lower denier, maybe 600. That way the waterproofing will at least keep her dry and she won't get to warm on sunny days. I may even go with that for my yearling.
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Denier doesn't have anything to do with warmth; it's the fabric strength. Fill, sometimes called polyfill, is the warmth factor. I keep several types of fill on hand- 100, 200, 300- and blanket according to the weather that day. You can also layer. I would go a little on the bigger side, but not so it's unsafe. What does she measure at now?
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I feel like a complete idiot. Guess I should look up that stuff before I talk. No wonder my winter blanket isn't lasting very long. I'm so dumb! Oh well, learn something new every time I'm on here.
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Haha no harm done :) I may or may not have worked in a tack store in my youth- all sorts of useless information stored up in this noggin; no wonder I can't remember anything now, my "RAM" is full!
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I personally wouldn't blanket them. They would do best with access to a shelter and lots of grass hay. What happens in the middle of winter and a blanket gets ruined or needs repaired and then they do not have another one to put on? So you should also have a spare.

She's a 55' now.

I thought that it was the change in sunlight that brought on a winter coat, not the change in temperature? My goat had to be blanketed every winter because the barn she was kept in had automatic light that where the same all year round so she never would grow a winter coat. And I have known horse farmers who set up automatic lights in their barns and time them to have winter time lights early/late/never so the horse will grow a coat when they want it too. Doesn't work for a horse that gets turned out all the time or a barn with big windows that let light in when you want it dark. I also thought weanlings/yearling and such didn't need blankets unless they are sickly.