Are they tiny lil root cuttings? If so; a 4in pot will do for over-wintering... If they are large root cuttings, then I'd suggest more like 8~9in pots...
I agree that the garage might be a good place to get them started; or perhaps on the floor somewhere in the house... Once the foliage breaks the soil surface, they will need some light. Given that your winter sun is even less than ours, I would place in a south-facing window that received "sun all day" (well, you know what I mean)...
After the last frost; dig a hole that is a minimum of 6in wide by 12in deep. Cut the foliage down to one inch stubs and place in hole. Back-fill with a (cool*) manure and a bit of good
compost. Place plant in hole 1~2in below soil surface. Add 1~3in of mulch on top of that (leaf mold mixed with small woodchips works super)
*Goat,
deer, llama, alpaca, rabbit or any other cool to semi-cool manure such as equine