Up until recently, I had a Deere LX280 equipped with a 48-inch deck and had the the Kawasaki V-twin rated at 18 hp (there was a lawsuit a while back alleging it was not really an 18 horse engine, so maybe it really wasn't---I dunno). I cut two acres, and it was not unusual to have to slow down quite a bit to get some of the thicker growth. There were many days it would take me almost four hours to complete the job.
More recently, I acquired a Deere 445. I believe the tractor is rated at 22 horsepower but it may actually be 24, I just don't recall. This one has a 54-inch deck. This tractor cuts MUCH faster and I rarely if ever have to slow down. I can complete the job in about half the time.
I would not want to tackle 5 acres with that old LX280, for sure. The 445, that would be okay once or twice, but I'd want an even bigger tractor for that job if I had to do it regularly.
Based on that experience I'd never consider going with an 18-horse tractor for a five-acre job.
