Install a terminal app, like
Terminal Emulator for Android. Run it and issue the command
df
Look for a line ending in
emulated or
sdcard. (It might be easier in landscape mode.)
That line shows all the storage you can access. The rest of it is in partitions for the system, you can't use it. I'm guessing that the sdcard partition has 1000000 1k blocks (the first column) or less. That would be 1GB. (Scroll up if you don't see the column headings.)
when you're done, issue the
exit
command, or the terminal app will keep running.