Bed size?

Would knowing what you want to grow for the next 5 - 10 years dictate the sizings?

Feel free to shoot me down if it's a stupid question

Not really. I think 2 foot is a good width for any crop either in a single row ie brassicas, tatties or in double or treble rows for onions leeks etc.
I like to grow a bit of everything and can't see a problem. As i said, this is just me thinking about what I am going to do and exploring options.