I have a couple of homemade cone shaped boxwood topiaries in pots on my front porch. I'm not an expert but I used Buxus japonicus, the Japanese boxwood that is common here in So. Calif. The plants should almost, but not quite fill the pots. If the pot is 15 in. across at the top and you put a 5 gal. plant in it, you have a few inches all around for it to grow. Make sure the 'urn' you mention has good drainage. I used two cement urn, about 12 inches. across, and planted two spiral topiaries in them. They were expensive plants. They died in those urns. You can keep them in the pots for years. Once they are root bound, though you should do root pruning and put them back in the same pots.