You can keep them to whatever size you find convenient, within reason.

Unpot and shake the excess soil out of the rootball. You may find that there was a lot of empty space in the middle of the roots and you can just put new soil in and reuse the same pot. If the pot is actually full of roots, go up a size (2" larger in diameter than what you were using before).

When the pot gets to the maximum size you want to lug around, you can rootprune annually to rejuvenate the plant and keep it suited to its pot (do a search for rootprune on gardenweb for help if you need it). Some varieties are easy to root for cuttings, so you can also start baby plants to eventually replace your big monster.

I have some varieties (mostly fancy hybrids) that seem happy in a 10 to 12" pot for many years. Others are huge (single red flowered garden varieties like 'President') and want much larger pots.

Thanks! Great advice--just what I needed!

I can never remember how the pot sizes are measured. Is that diameter?

Thanks again!

You're welcome. The sizes given are the diameter across the top.