I notice you said JD650, which has a 2cyl engine with about 16hp at the PTO, but to the left you listed a Yanmar YM-1500. They probably share the same engine, and maybe most other components?? Anyway, the size of mower it can handle will greatly depend on what type of cutting you will be doing with it. If this is your yard you're cutting and you're planning to keep it mowed on a very regular basis, then it will probably run a 5ft finish cut mower ok. If the grass is wet (lots of rain, heavy dewfall in the morning), it can clog up the bottom-side of the mower deck and make it harder for the mower to discharge the clippings and make things worse. That will put some strain on the engine.
If you're planning to cut a yard (or a field) with it 2 or 3 times a summer, then it will probably struggle to cut it while making full width passes with a 5ft deck, because it will grow pretty tall in between the cuttings. In this case, a 4ft deck would be the better choice. Just some things to consider in deciding which size to get.
I will add that there's nothing worse looking than a yard that was cut with a mower not up to the job and it leaves the grass cut unevenly, and there are big clumps of grass everywhere. I guess I'm a perfectionist when I mow the yard, but not so much when I'm bushogging, don't care so much what it looks like then. :laughing: