From my experience i have found that if i use commercial beer that is already carbonated, you will be looking at approx 7g/litre of co2 to dispence. If you want to carbonate, it's an additional 7g/litre. (based on a kegerator system)
So if you are home brewing and you system is free of leaks, you should be consuming 14g/L or in other words you should be getting 71L for every kilo of co2.
These figures are based on use with a kegerator, PUB systems use a little more because they are generally running at higher pressures to lift the beer up from the ground floor or basement, or keg room, cellar etc.
The cheapest place to purchase your cylinders is from www.kegking.com.au or one of Keg Kings distributors, but i'm a little bias because I am good mates with the guys there. But it's a pretty good deal. They are $190 and they come with a lifetime warranty and servicing. So you will never have to pay for hydrotesting or replacement of valves and burst disks.
2.6kg cylinders are a good managable size and fairly safe. The size of the cylinder should also be chosend to suit where you are using the system.
If i had a confined bar room with little ventilation i would definitely not use any more than a 3kg cylinder. In the case of a leak, a concerntration of more than 30% co2 can be lethal. 3kg will be enough to make a 50cubic meter room a lethal environment.