The rule of thumb is you should use acetylene at a rate that is not greater than 15% of the tank in an hour. So if you use acetylene at a constant rate, you should use a tip small enough to get 7 hours use out of the tank.
If you really need to crank a bigger flame for a moment, that is not a problem. Sustained use at higher flow rates is the real issue.
In the tank Acetylene is stored disolved in acetone. As you draw acetylene gas out of the top of the tank, more aceylene comes out of the acetone and goes down the line to the torch. At some point you'll consume actylene faster than it can be replaced.
At that point, you start to draw acetone into the torch. (one of the reasons never to use an acy tank on it's side) Acetone in the torch will cause sputtering and some welding difficulties, but you have just burnt up some acetone. Acetone holds acetylene - so by losing acetone, the tank will now hold less acetylene.
As to tank size, I recommend the largest tanks that you can move around yourself. There was a welding shop around here, big fire, lives saved because the first thing they did was get the acy tanks out of the building!