Just how big is this field?
I run a 8' twin behind my MX5100. Which is rated to lift less, and less power, but same basic frame/platform.
Don't take the comments "you should be able to run a 72" cutter" as meaning it's borderline. Because that's how I read it weather it was intended that way or not.
6' would be the MINIMUM I would want.
That said, back to the acreage question. An 8' twin cost about double a 6' if shopping new med duty. And in the used market, 8' twins you rarely see if ever, 6'....much more easy to come by.
If you have small acreage, like 5-6 acres, I'm not sure one can justify the added cost of the 8' mower.
Bit if you have 15-20 acres that you are going to be mowing with any regularity, I'd look for an 8' twin. And if hauling it around on a trailer isn't going to happen, I'd even be inclined to look for a pull type 10' mower