> What I would like to now is, roughly, how much disk space will that take
> up,

On my SE/30, the filesystems /, /usr, /usr/src, and /usr/X11R6 are
using about 19M, 53M, 200M, and 74M respectively (/usr/src includes
compiled objects and executables).

> and how much of the tree can be shared between the two.

Almost all of it!  Not just source tree, but most binaries are
sharable between the two.  Exceptions are X servers and few others.

> (assuming NetBSD/mac68k will recognise my Asante card - gulp!).

I think the Asante card will work.

> Also, are the two 68k ports binary-compatible?

All m68k ports except hp300 port can run same binaries.

Hope this helps,

ken