I disagree with the advice so far. Your question is the problem. It is like someone saying that they want to be able to do 250 foot scuba dives sometime in the future so what size double tanks should I buy now for my first scuba lesson?
A 40 might be fine for a stage bottle for technical diving, but it is way, too big and heavy to drag around on most recreational dives. I know lots of people that have 30 cu-ft tanks but they are such a pain they won't use them on a 110 ft dive. They get left on the boat or in the garage.
For no-deco, recreational diving, I prefer a SMALL, pony, one that will get me from the bottom to the surface and maybe give me a couple minutes of time to hang at 15 feet. For this type of diving, you only need a 13 or 19 cu-ft bottle. You need to balance safety with inconvenience, or otherwise you would be carrying an 80 cu-ft pony bottle clipped off to the shoulder, which MUST be safer right?
You need to do the math on your own breathing rate, your expected ascent rate, if you want any reserve to handle problems on the bottom or do you assume an immediate ascent should your have a total failure of your scuba unit.
Get a small pony bottle now and a bigger one later. If you get into technical diving in the future, the cost of another stage bottle is NOTHING.
Today i was diving solo in about 95 feet, I was carrying a 6 cu-ft tank, which for me, is enough to get me from the bottom to the top and this tiny tank has no noticable drag and the weight with a reg is like 5-6 lbs, I guess.