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I travel and dive with a 19 cuft pony and use it within recreational depth limits and no deco. With a 25% increase in my SAC, a slow ascent, and a 3 min stop, I would use about half the tank. With a 150% increase in my SAC, I would use nearly the whole thing. I'm fine with that.
I don't think it's a good idea to be diving below 100 ft or so without understanding something about gas planning. At a minimum you need to know how much gas it would take to make your controlled ascent and safety stop from the depths you are diving. I make sure to always have enough gas for both me and my buddy to return to the boat and I expect the same courtesy from my buddy. Search for rock bottom.
That said, you only need to account for one diver for determining the pony size since a pony is for self-rescue. From rec depths a fully pressurized 19 is probably fine. I preferred a safety factor and did not want to count on the pony always being full. That bumped it up to 30. Since I was planning to go tech, I just went with a 40 which is now a deco bottle.
Whatever you go with, practice is key. I wouldn't carry anything with me that I didn't know how to use- unless a purpose of the dive is learning how to use it
A ScubaBoarder came up with an excellent Excel spreadsheet for calculating pony capacities necessary given depth, consumption rate, reserve time, safety stops, etc.
I can't find his original post, but I referenced it in another post here:
So play around with the numbers and see for yourself. For many of us recreational divers who typically stay less than 100' down, the 19cf seems to hit the sweet spot between capacity and portability, but 13cf, 30cf, or 40cf also seem to be widely used.
Depending on depth and water conditions Cold water warm water?
Compare the 40cf with that that of the 30 of 19.
If with the 40 upon return to the surface you have enough to return to the surface twice than a 30 will be sufficient under the same dive profile if not than a 30 is not suitable and the 19 would be out of the question.
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