By coincidence, I spent almost 2 hours this evening repotting my Onc. maculatum. The plant was originally potted in a 5" pot about 3 years ago. Other than watering and fert. I did not pay too much attention to it untill it gave me several spikes this year and won an award.The thing had grown all over and down the sides of the pot so that in order to show it I had to use a very large decorative pot to place it in. Now that the spikes are gone I decided to divide and repot. However, just as you said, I want to have a large specimen plant.
I have kept a large part intact and potted in a 10"w. x 5"D plastic pot, (easier to get out if you need to repot later). I also got a smaller piece which I have put into a 6" pot.
I am sure some will tell you not to go for such a large pot, but I prefer to wait a while for the plant to grow in rather than have to repot often.