What where the historical forces at work, the tradeoffs to make, in deciding to use groups of eight bits as the fundamental unit ?

There were machines, once upon a time, using other word sizes, but today for non-eight-bitness you must look to museum pieces, specialized chips for embedded applications, and DSPs. How did the byte evolve out of the chaos and creativity of the early days of computer design?

I can imagine that fewer bits would be ineffective for handling enough data to make computing feasible, while too many would have lead to expensive hardware. Were other influences in play? Why did these forces balance out to eight bits?

(BTW, if I could time travel, I'd go back to when the "byte" was declared to be 8 bits, and convince everyone to make it 12 bits, bribing them with some early 21st Century trinkets.)