4" pipe works, but you won't be able to tumble much brass at a time. I started with a 4" pipe and can still use it for small batches. I now have a 6" pipe that holds a little over 2 gallons and can tumble a lot all at once.
I found a company local here that sells 6" pipe by the foot. Also, your local city probably has drop scraps of 6" pipe. I happened to find the guy who hauls scrap for the city and got several 2 foot sections of 6" pipe. Our lowes also carries 2 foot pieces of 6" green pvc pipe. It is about $16 each, but still less than a 10' stick.
--Daniel