I use one for solo diving to around 100 feet..
He is being sarcastic, of course.
At the end of the day, everyone has their own idea of what is "enough." You will never find a straight answer, because it's all relative. If you only dive in 30 feet of water with straight access to the surface (no boat traffic etc.) and with no entanglement hazards, a 6cuft pony is plenty.
However, some will say even a 40cuft bottle is not adequate, and they may be right, for example if you do solo diving in deep water and difficult conditions and/or entanglement hazards.
I spent a lot of time researching my options after I completed my solo diving class, and I decided on a compromise that would work FOR ME. That doesn't mean it works for everyone else.
I decided on a 19cuft, because I wanted it not to get in the way of my photography (I lug a big rig around underwater), and be able to take it on air travels, yet still with enough gas to ascend safely from 120ft+. I tried it once from 100 ft, did a nice slow ascent on the anchor line, a ten-minute safety stop, and if I remember correctly, still had 1000 psi in that little 19 cuft bottle on the surface.
That being said, had this been a real out-of-gas scenario, I would have been considerably more stressed out, breathing rate up, no anchor line and perhaps fighting a current. Would 19cuft be enough in that scenario? Maybe yes, maybe no. If you add entanglement to the mix, you can easily see where this is going.
But based on my scenario, I feel confident that I can make a reasonably slow and safe ascent from 130 feet including a good safety stop, on my 19 cuft pony.
In the meantime, though, I was diving with a 40 cuft bottle on one trip, and found that it hardly got in the way more than my little one. The reason is that when you sling your pony (IMHO, the only reasonable way of carrying it, but again, that's just me and you will find all kinds of differing views on that), the bulk of the bottle extends up next to your body and you don't even notice it.
My 2cents: If you want to travel with it, get a 19cuft (I consider that the absolute minimum). If travel is not an issue and you can see yourself doing serious solo diving, I wouldn't go below 40 cuft or at least 30 cuft. The 30s are actually a nice size, they're much lighter than a 40 cuft, but don't quote me on that. That's just from remembrance, haven't looked up the numbers. Also, if technical diving is in your future, a 40cuft can make a good deco bottle. A 19cuft is a bit less useful in that regard, except for 100% O2.
I wouldn't buy a pony without knowing my SAC rate. I determined mine as part of the solo diving class. I sat in my apartment, breathing from a tank for 30 mins and wrote down my air consumption. Now, this of course will give you a very "benign" number, and you will have to adjust it according to circumstances. For example, for contingency planning, I would double or even triple that number to account for stress. Then multiply the result by whatever ambient pressure you're doing your scenario for (e.g. 4 atmospheres at 100 feet), and how much time you want it to last. That will spit out a result that would make your best guesstimate on how big you want your pony bottle to be. You might be surprised and find that you need to go with a fairly big tank if you want it to cover a light entanglement scenario at 110 feet under stress AND a safe ascent after that.
Good luck!
