I had formatted a usb flash drive to be a linux boot disk. Then, when finished, I went to reformat the drive and was only prompted to format 168MB out of the 1GB it originally had.

Open up the command line and type in the following:
disk part
list disk
select disk YOURDISKNUMBER
clean

After cleaning, you should be able to right-click and format again with the full amount of space. You can also finish this up using command line:
create partition primary
select partition 1
active
format fs=fat32 quick
assign
exit