It's likley 1/2" pipe, which is about 5/8" OD. You can buy a ferrule puller. It sort of looks like a gear puller which should pull that ferrule off. Sounds like you had a defective ferrule. Kind of strange, unless it was made in China - they have problems with tempering and alloying metals.
If the ferrule actually split, you should be able to wrestle it off without a puller, though. You could have split the nut, too, but if so, you still need to get the ferrule off to put a new one on. The compression fitting doesn't need to be super tight. It needs to be tight enough so it doesn't spin, but not so tight that you compress the pipe so it is crimping the pipe a lot.