Minimum is something that can carry a little more than 2 combine hoppers. I'd go with a 1000bu. That way you can fit two full combine hoppers unloading on the go (might get up to 400bu each by the time they dump if they're full when the cart arrives) and then you also have room to carry a few bushels back into the field if you're not able to dump fully at the trucks.
I think 1500bu is overkill for just 2 combines. I get the attraction of bringing full semi loads to the truck, and the 4wd/cart is doing fewer trips to the gate, but then you're always pulling really heavy loads where you're causing compaction, more stress on the tractor when bouncing across the field, more likely to get stuck if it's wet. The exception is if you're doing a lot of long hauls with the cart or if you're planning on getting a 3rd combine... or if that money is really burning a hole in your pocket.
I ran a 1000bu cart for 3 class 9 combines for a few years. The cart was a limiting factor only in the cereals when the hauls to the gate were longer than 1/2 mile, or we were in perfect combining conditions. It was often enough that for 3, I'd get a 1200bu at least. A 1000bu is fine for 2 (if cost is still a concern!).
For topping off the cart, the easy way to do it is have the cart maintain a steady speed and direction (a little slower than combine capacity), and the combine operator moves back or ahead. Absolutely get a PTO cart, they dump way faster than hydraulic.