I am often conservative in sizing implements, especailly for other people due to the fact that each of us has our own particular conditions. You said you have some clay, I would suggest that you not go any larger than a 48" box blade. I have a 5' box that I use with a 30hp tractor and I have heavy clay, it doesn't stop the tractor but it will occasionally grunt and groan. I've used the 5' box blade with my 24hp tractor and had it stopped several times with the tires just spinning. But my ground is really heavy clay. I just got my new 48" box blade and have not tried it yet. But with my experience, I'd say 48" is the right size. If you have sand you might be able to go bigger. If you are not using the scarifier teeth much, then you can go bigger.
As for the rotary mower, I'm not sure what you mean. Generally a rotary "cutter" is a heavy duty mower for clearing fields. A "finish" mower is a large grass cutter used for maintaining lawns. Are you looking for a lawn mower or a field mower? Either way, I'd be looking at about 48" units given that you have roughly 15 pto hp. You might be able to use a 60" finish mower deck, I can choke a 60" deck when I hit a patch of thick clover on my 24hp tractor, so it really depends on how lush or sparce you lawn is. There are light duty 60" rotary cutters, but realize they are not going to be very effective at cutting down saplings that are 2 or 3" diameter with that amount of available PTO hp. Generally, you would use a smaller rotary cutter than you would use a finish mower. Also, my personal disclaimer, I am not a fan of buying large implements that force me to "go slow" or take "half cuts" because that means it would actually take me longer to do the job than if I properly sized the implement to the task. Consider you conditions and your tasks when following anyone's advice about the size of the implement to purchase. We all have different experiences.
