If you want to do much more than just boot Linux/m68k you will need 60 MB or more of free space on your hard disk and a supported hard disk controller. Add another 20 MB of disk space to the base requirements to use X.
In addition, you'll need some swap space. Any amount will do (and you will need less if you have more RAM). 16 MB of swap space is a fairly reasonable size for most systems; the kernel is limited to using 128 MB of swap space.
Due to the general law that your files will expand and multiply to fill your available disk space, I recommend getting the largest disk you can afford (and your system can handle reliably).