rootfs_linux_v1.4.ex4 doesn't boot?

Hi.
would someone tells me how much the "xxx" should be for making the "rootfs_linux_v1.4.ex4" and "root.ex4 for android" using the mkext4fsimage -l xxxM  command?
I made both of them without errors but after flashing them none of them boots and the following errors happens:
Kernel panic - not syncing: VFS: Unable to mount root fs on unknown-block(179,2)

You can find these in the Development Document(but in chinese)
linux filesystem is 256M
And android filesystem cannot being made like linux.
You can only install android by the SD card's method.

Hi.
Thanks a lot for your helps.