You can check the size of the flash memory in Information on your device or properties on windows explorer.

Let's say that the physical available storage for your device is 256MB (= 256 x 1024 x 1024 Byte.)
(FYI. 1MB = 1024kB = 1024 x 1024 Byte.)
However, some space is shared with firmware and file system for the removable disk.
So, the available storage that users can use is less than 256MB.
Let's divide 254,xxx,xxx Byte by 1024 twice. Then it is around 243.

It is a common situation for all flash memory type of MP3 players which is also usable as a removable disk.

Thank you.