It is going to depend a lot on how protected the space is. And really the more insulated and weather tight, yet still vented you can make the place the heater is going in, the more comfy the critter will be be using it anyhow.

For our barncats, we have one by NAMsam that is 24" by 18". Key things I was looking for included adjustable temperature and an armoured cable since they were still kittens when we got it. We first just had it in a wicker basket, with a thin blanket over it because they have filthy muddy paws plus the cover super attracts fur. Much easier to clean the blanket. They are a bit big for the basket now, so we built them a box with the bottom the same dimensions as the heater and insulated the top. The bottom and half way up the sides (like a baby crib bumper?) we made thin cushions so they have a kinda couch thing. The other sides are open so moisture doesn't build up, but when it is super cold I drape a blanket over it. Front is cut out a bit to allow them to go in and out and thermoregulated by draping half in. They have a towel covering the pad and their thin blanket which they have turned into a little nest in there.