I can't see anything in the documentation explaining what the capacity of the Belt Station is supposed to be, only a checkbox for "bigger cargo waiting area". So what's the normal size and the bigger size?

I'm using a short length of the conveyor between two railway stations separated vertically by 600 metres, so the high level Conveyor station needs to have capacity for multiple trainloads being transhipped from train to conveyor, and the low level needs capacity for conveyor to train. The peak load could be as high as 1,200 units.