From the combine operators standpoint I like one that will hold at least two and one half combine dumps. This makes it easy to dump two combine dumps on the go without having to top off. How quick the buggy unloads and if the ground is wet determines if you will be emptying the cart every combine dump or every other. Even if you are dumping the buggy every combine dump it is nice to have some extra in case a truck doesn't make it back quite in time.

This is assuming you have enough trucking capacity. If that runs short sometimes then a bigger cart is nice. If you are using it as a holding tank having one big enough to fill a semi is nice but that is a lot of added weight to drag around all the time if keeping the combine going is your main goal. A compromise is having one big enough that the cart and one combine can fill the semi. If the cart is full because it is waiting on a truck then when the truck arrives the cart and combine have it on the road again pretty quick. You can even dump the combine in the buggy while it is filling the semi so the combine can get back at it.

 In your case the size of the buggy is going to be limited by your available tractors and budget. Probably 500-750 bushel in your case. Tires make a lot of difference how one pulls, especially in soft soil. We run Super Terra tires on a Brent 874 which would be expensive to buy new (we kept them from a pervious combine) but the smaller terras  and rims can be bought pretty reasonable used sometimes in a dry fall. Makes a world of difference in soft ground. We ran a Wetmore 400 (same as older JD) cart for years and when we took the 14:00 x20 tires off and put a set of worn out used terras off a Gleaner combine ($800) it made a completely different cart out of it. With the small tires our JD4450 or White 2-155 2wds at the time would just spin out if the ground was just a little soft. With the terras on the cart the front tractor tires would start pushing mud before the cart would sink with one combine dump. It went from an "end of the field holding tank" to an "unload on the go" usefull tool.

John