Any one aware what is the disk size on AP-303H running openwrt firmware? https://openwrt.org/toh/aruba/ap-303h shows Flash MB as 128NAND. Does that mean it overall get 128 MB?
No it's the flash chip size.
You could try to figure out the flash space you'll get by reading the flash layout chapter on the page you linked to, seems Openwrt is written to mtd16.
'Doable', but pretty cumbersome and inconvenient when it comes to (sys)upgrades. I wouldn't recommend extroot, these days you can usually get devices with 'sufficient' amounts of flash/ RAM which don't require weird hacks to function.