The best engineering answer: Depends. Depends on what hardware/architecture is implemented. Usually CPU's use a certain size for an instruction set and decode with multiplexers to perform the intended operation which will expand the bits. Simplest answer, is 64bits or more of BUS width would be needed for a 64bit instruction.
Allows for a wider instruction set and essentially more processing power as more data can flow through at a single time versus having 32bit hardware limiting you to a 32bit wide BUS.
No, he would need to speak to an EE or Computer Engineer, both of whom deal more closely with hardware and architecture of such things.
Software engineering is geared toward optimization and implementation of code. The only thing "hardware" related really that SE's worry about is parallel processing - and even then it's just using threading libraries and such and making sure your variables aren't going to run into a deadlock situation or that threads aren't modifying each other's values.
Unless of course you're programming in a very old environment and using straight up assembly, then you would be more mindful of what CPU you're using because then you're directly limited to what registers on the CPU you can use/access because Intel and AMD have their own special registers they have on their chips. Compare this with ARM CPU's who has an even different instruction set and architecture and what registers and such you can use.
