RAM and MMU requirement in ARM 11
ARM11 is an architecture. It contains an MMU. The ARM architecture itself doesn't have any RAM at all, that is dependent on the Chip you use (for an example I will assume an i.MX31 which uses the ARM11 core) and the amount of RAM you can interface via its memory controller. I know that the i.MX31 supports 256 MB (maybe eben more). Therefore this should be enough for a Linux installation.

You won't port Linux by yourself to such a chip, as this will usually be done by the chip vendor (or some subsidiary). Freescale (the vendor of the i.MX31) provides a board support package for both Linux and Windows CE (see section Run-Time Software). Of course, you are free to make improvements or changes to that basic board support package.