What size heat pad do I need for a small sized whelping box?

unless these are going to be born outside, they don't need a heating pad. Momma's udders will get very warm, almost hot and she will keep them warm.

Never used one.

Measure the width of your whelping box and that should give you the answer. We never used heat pads - brood lamps give a more overall warmth to the area - and dry damp which is bound to happen. We had a thermometer on one of the pig-rails to keep a check on the heat which ideally shouldn't be lower than 75 - 80F. We did have to drape a sheet across the back of the box, so mum could get out of the direct heat from the lamp, which has to be hung at a sensible height above the box floor.