iSi Gourmet Whip Plus - 0.5l or 1l?
I have an older 500ml/pint version and a family of 4; I find it more than adequate for my applications. If I'm doing whipped cream it's somewhat unusual that I go through it fast enough, so I can imagine it would not be helpful to have a larger one unless I was using it for catering or extreme holiday cooking.

For what it's worth, I usually find half-full the minimum useful fill line on the 500ml. (which is to say, about 250 ml). Some people really insist it needs to be full. The cream tends to get too aerated and not dispense properly if you use it for too small of an amount, plus you're going to use at least a full cartridge per use anyway, so it's kind of "wasteful" to use less considering the cost of the disposable cartridge.