It DEPENDS!
What is the mowing surface like and what is the stuff to be mowed like?
I'm running an 7'+ flail on my Kioti5510HC: engine 55hp, PTO 43hp (I believe). Manufacturer recommends a minimum tractor of 60hp. I took the risk because my grass isn't nasty thick stuff (though it can get really tall); also, my ground, though pretty flat, is bumpy as heck and no matter what you're doing you're NOT going to be running fast on it[AND, there's a LOT of trees]- I am running in low gear with the flail)- reducing speed reduces the required applied power (you can go out and cut stuff up by hand, but it's going to take a LONG time!).
I'd say that the 6' flail isn't out of the question as far as being in a general spec-range. I'm a big fan of hydraulic side-shift, so if you can get it and you think it would be something of use to you then get a flail with it.