I am running a full size Hesston 4550 square baler with 26.5 PTO HP at altitude with little issue. Just need to slow down. Look at my signature and search for the many posts about this topic. I have several out there. Now if your just starting out and buying your first tractor, you need two things Weight 3600lbs or more of tractor and if possible above 35PTO hp. I agree 40 would be better. Hesston has a round bale unit that takes only 30 HP to run. Keep in mind - many HP ratings are to get enough weight of tractor to handle the implement, not what HP will actually run the unit. Tip - research the implements you want to get and determine the HP requirements and weight. That will tell you the lift requirements of the 3pt and HP needed. Last get as many set of remotes as possible on your purchase. They are cheaper at first (if buying new). You may not think you need them now, but at some point you will.
Thank you for your input.
Hope this helps