The best size for mending a 22ct wedding ring is the smallest, and if you want to melt the tarmac across your drive, the biggest. obviously.
Or, more realistically, it is the flame size and intensity which counts. So, work out what you will need to do with your outfit, and get a range of sizes. Not every one in the catalogue, but three, perhaps. And when you need something especially delicate, or beefy, consider adding another.
You are also limited by the size of the adaptors, the cylinder connection, the pipework, and – yes – the temperature. On a cold day, the gas (liquid in the cylinder) evaporates more slowly, limiting the max output.
And don't knock your cylinder over – it is not clear to me how a pressure regulator will stop liquid (at the same pressure) from flame-throwing.
cheers, Tim