I have 2 Muscovies. It took about 2-3 months for the eggs to become consistent in size, but the eggs started getting bigger at about 1 & 1/2 months.
When they first started, I'd get a narrow egg one day, then a fairly round egg, and all in between, I've had huge ones which seem to be a mix of...
My hens eggs are fat and can get up to twice the size of chicken eggs, so if I fill a carton full of their eggs, I often can't close it.
When they first started laying the eggs were long and narrow and were the size of a chicken egg, so I could fit them all in a carton with no problem, but...