Note that your gas vendor will probably be willing to later let you trade-in a small cylinder toward credit on a larger one. Starting with smaller set of cylinders could allow you to get going at relativly low cost for now and still be able to size-up later if you need the extra capacity.
That said, I have a portable "peanut" rig and seem to run through acetylene about twice as often as O2. It would be nice to have a larger acetylene tank so that I could expend both tanks at close to same time, fewer trips to the gas man to get just a HC tank refill. If you dont have an open bed truck, it is quite a hassle to borrow one transport cylinders. Dangerous and illegal to transport them in a enclosed car and most gas places (rightfully) will refuse to sell to you without safe means to transport. Before I had a truck, I used to remove the trunk from my car in order to transport cylinders.