Ha!!All I can picture is lining them up to see if they would fit. Very cartoonishly. LOL. I’m a very visual person. Haha.
Best to have it a bit bigger than they need, the larger opening is so none get trapped in there if too warm. It emulated the heating plate, they feel safer if they can get under something. You could actually put the plate out there unplugged, they'll go under cause that's what they are used to.
They'll stay plenty warm as long as there are not strong drafts....and hopefully won't get too hot during the day, will they have access to a run?