>    - How much disk space do I need to create an environment
>      where I can rebuild my kernel?

My kernel sources at 14 meg.  That includes the i386 and sun3 machdep stuff.

You will also need ~2 meg for objects.

>    - All I want to do is add support for an A2232 board.  How much
>      of the source tree do I need?

All of the generic stuff (although you probably don't want any net stuff
other than netinet) and all of the Amiga stuff.

You need to compile all code that your kernel currently uses.

>    - Any other questions about configuring a new kernel which may
>      not be obvious to me at this time...

Simply RTFM.  The FAQ should tell you how to build a kernel.