It is possible (but very difficult) to boot Linux/m68k with as little as 2 MB. Now you know that the kernel works on your system: that's it. If you want to work with it you should have at least 4 MB (8 MB is probably the working minimum if you want to run an X display on your machine); booting from most ramdisks will require at least 5 MB.
On the Amiga only the size of your Fast RAM is relevant. The Chip RAM is only used internally for graphics, sound and floppy data; it is not used by normal programs.
If you have an Amiga with 16-bit expansion RAM on a Zorro card, see the section called Amiga with GVP 16-bit RAM in Chapter 6.
On Ataris, ST RAM is usable on most systems.