Follow these steps:

      1. Run cmd.exe(command prompt) in administrator method.
      2. Type DISKPART Enter.
      3. Type list disk Enter to list all disk and note pendrive disk number(example disk 2).
      4. Enter select disk 2.
      5. Enter clean.
      6. Type create partition primary Enter.
      7. Enter exit to exit from cmd.
      8. Go my computer select your pendrive and format. Now you can get full size of your pendrive.

Example screenshot:

Note: If any data is there in your pendrive you should take a copy before do this. Because all data in pendrive will be erased in this method.