Pair of geese overnight
How high they like to be able to stand completely up and it also makes it easier for you to for cleaning if you can walk inside. 6X4 would be plenty big enough for 2 geese.
Sounds perfect as long as secure from preds!!
3 would be fine but if your looking to add goslings you might have to build on eventually. Geese like their space. lol
this is what Holderread says about floor space overnight shelter should have at least 7.5 to 10sq ft of floor space per bird.