More is better, of course, but there is no real lower limit (though “at least” as big as your RAM is necessary if you intend to use it to hibernate*). I think a good approach would be to put the bootloader and the kernel itself on the eMMC, and use the remainder (should be over a GB) as swap. /home, /var, etc. could go on the SD card (or just use the SD card as regular external storage, and never save anything to the regular fs).
* ooooohhh great, now I have to figure out if I can set up wake-over-lan…
Where are all these coming from???
Korea
And how are you finding these obscure ARM computers?
Google
