I run an 8' (7' working width) 1,700lbs flail behind my L4060.
A lot of it will come down to how fast do you want to go & how dense is the material. I'm a fan of wide & slow. Leff bouncing around for me & the machine going slow. Wide means I'm mowing the same acres per hour despite going slow.
Kubota L4060
Engine (net): 40 hp [29.8 kW]
PTO (claimed): 34 hp [25.4 kW] (gear)
32.5 hp [24.2 kW] (hydro)
5 horse power per foot of length = 40 horsepower. If the tractor is a gear tractor you are 6 horsepower short of recommended horsepower. If a Hydro you are 7.5 horsepower short.