I'm assuming you mean FAT32. What OS are you using (Win XP, Win 7, Win 8, etc.)? Were you formatting from a command prompt? If yes, what did you type in the command line?
Just out of curiosity why does the flash drive need to be formatted as FAT instead of FAT32? This may help me find an answer for you. There is a partition limit of 2GB when formatting with FAT which is why you are getting the "volume is too big" error message.