I received a moded 703n (8M/64M). This guy simply used original 4M uboot and it works fine with my 8M rootfs. What is the difference between 4M and 8M uboot?
I figured this out. A 4M uboot cannot write anything longer than 4M. However, the Openwrt can detect flash memory size on its own. Thus, to flash a rootfs larger than 4M, one need to first install a 4M rootfs and then use ssh or luci to install a new rootfs, which can be larger than 4M. A two step process but not too bad.
Aha, that is some useful information! Could you please explain how you went by to extend the rootfs "live" without uboot modification?