The size of the physical FLASH is definitely 512KB. CircleOS + FAT consumes the first 24KB, plus the last 128KB. The rest is consumed by the applications that are already loaded. You can restore the original firmware, without the applications. You can also remove CircleOS and program your own project without CircleOS, and you will have 512KB. Note that the FLASH used by CircleOS contains the 'welcome message', and the bitmap of the home page... that's the main part.