You can add pretty much anything you want to the back, as long as it's offset by weight in the front.
Theoretically there's no real limit as long as it's well balanced from to back. But in the real world it'll get harder and harder to steer the more weight you add, and your tires are only so wide so they'll start to sink more and more, and your brakes will work less effectively - especially on hills.
So I don't think there's a hard and fast number to give you - the answer is more "it depends". Err on the side of lighter.
Aside from the weight - there's the torque required to spin a big mower. Sure an A could spin a 6' mower - but through what? 6" high Kentucky blue grass? probably. 1' high wet pasture - probably not so much.
I wouldn't go more than 5' - and that's assuming you're doing grass - not using it as a brush hog.
keep in mind too that a big mower is going to continue to drive the tractor even with the clutch in - that surprises people, often at a very bad time to be surprised!
If you're using it just for grass - I'd also consider turf tires to avoid leaving tracks everywhere.