You must have at least 32MB of memory and 110MB of hard disk space. For a minimal console-based system (all standard packages), 250MB is required. If you want to install a reasonable amount of software, including the X Window System, and some development programs and libraries, you'll need at least 400MB. For a more or less complete desktop system, you'll need a few gigabytes.
On the Amiga the size of FastRAM is relevant towards the total memory requirements. Also, using Zorro cards with 16-bit RAM is not supported; you'll need 32-bit RAM. The amiboot program can be used to disable 16-bit RAM; see the Linux/m68k FAQ. Recent kernels should disable 16-bit RAM automatically.
On the Atari, both ST-RAM and Fast RAM (TT-RAM) are used by Linux. Many users have reported problems running the kernel itself in Fast RAM, so the Atari bootstrap will place the kernel in ST-RAM. The minimum requirement for ST-RAM is 2 MB. You will need an additional 12 MB or more of TT-RAM.
On the Macintosh, care should be taken on machines with RAM-based video (RBV). The RAM segment at physical address 0 is used as screen memory, making the default load position for the kernel unavailable. The alternate RAM segment used for kernel and RAMdisk must be at least 4 MB.