For this thread, but relates to the threads about changing to a true read-only base file system plus one or more read-write overlays;
The space on flash required to store the base file systems:
mszick@core2quad /bun1/Builds/Kindle/k4nt $ ls -l
-rw-rw-r-- 1 mszick mszick 367001600 2012-03-02 06:56 mmcblk0p1.img
-r--r--r-- 1 mszick mszick 105803776 2012-03-02 06:56 mmcblk0p1.sfs
I.E: A real, read-only base system and a 250Mbyte read-write overlay would easily fit in the same sized flash partition.