Hiya

They should be able to stretch out, and have enough room.....have a look on
here to get a rough idea of sizes.
Whelping Boxes

Some can be very pricey - i got mine from here....

Pet heat pads and dog whelping box and whelping boxes from £20.47 Delivered Tel 01273 588511

I got a small one, but i think the opening is going to need to be trimmed down, so molly can get in and out with out hurting her boobies xx

but i know a lot of peeps use card board boxes. And cut them down to size.

Good Luck xxx