I'm developing my own theme. Does WordPress recommend or require a minimum or maximum when it comes to the width of the content or sidebar elements of an post, page or archive page? If not, are there any unofficial guidelines?
There are no recommended sidebar widths, however there are LOTS of plugins that make assumptions about this (especially those that handle advertisements).
If you plan on running ads, please check the ad width requirements and use that as your baseline. For example, if you are running AdSense ads and the ad requires 300px, then make sure that sidebar has a minimum width of 300px or your might break your layout or the ad might not show.
This can be tricky with responsive or fluid layouts, but being aware of it is half the battle.