Shearling and aviators
Shearling is a sheepskin tanned with the fleece left on, so the lining and the shell are the same hide. There is no seam between warmth and weather: the fleece faces in, the nappa faces out. It is the warmest thing a jacket can be made of, and it is heavy, which is why it is cut on the aviator pattern that was built to keep out wind at altitude.
What to expect
An aviator is roomy by design, with a wide fold-over collar that buckles closed across the throat, deep fleece cuffs and a body that sits at the hip. It is a winter coat that happens to be a jacket. Expect it to weigh two to three times what a biker does, and to be worn on days when nothing else would do.
Sizing
Shearling does not stretch and the pattern is cut with room for knitwear, so take your usual size unless the fit note says otherwise. Do not size up to fit a jumper; the room is already there. The collar and cuffs are the fleece of the same hide, so they add warmth without adding bulk at the neck.
Care
Shearling wants air: a wide hanger with space around it, never a garment bag for long. The fleece is brushed with a soft brush; the hide is wiped with a barely damp cloth and dried away from heat. It should never be conditioned on the fleece side and never machine washed. Have it professionally cleaned when it needs it.