I don't know what 30hp tractor you have, or what the available PTO hp is, but if the PTO hp is roughly 24-25 hp, then I would be looking at 60" tillers, give or take a couple inches. Now that said, I think SOIL CONDITIONS make a HUGE difference in what is the proper sized tiller for the available PTO hp.
If you have light sandy soil, you probably could use a 72" tiller most of the time. You'd probably still have to slow down in heavy spots, but it probably will work.
If you have heavy clay soil, then 60" is about the maximum size I'd be looking at, and perhaps even a bit smaller.
Now I am making a couple assumptions here. First I am assuming you want to run the tiller at full depth on the first pass (that is what they are designed to do, but many people buy bigger tillers than their tractor is designed to run and end up making 2 passes to get to full depth). Second, I'm assuming you know your soil conditions.
I would also point out that some "gear" tractors run pretty fast in 1st gear and are not suitable for tilling, especially if you get a bigger tiller. If your tractor is hst, you can go slow enough.