I run a 10lb CO2 cylinder on a 190 fully planted 24/7 with a Milwakee SMS 122 ph controller. I change about 40 gallons of water daily, so I go through alot of CO2. I have to refill the CO2 cylinder every 2 months.
What you select depends on where you plan to keep the cylinder too- a 5 lb may fit nicely in your stand, but a 10lb cylinder may not fit. I lucked out and found a short 10lb that just makes it inside of my stand. (Measure the max height before you shop.)
Check the local home brew, paintball and welding shops for rental / exchange / refill prices. You have to first have a cylinder for them to start you on the rental program to avoid a big down payment.
If not try:
www.beveragefactory.com or a local praxair.