[Q] How do I resize the partition that stores my data?

I own the 64gb version of the Oneplus One but after flashing cm-12.1-YOG4PAS1N0 only ~12gb are available. Flashing a different rom and reflashing cm-12.1-YOG4PAS1N0 didn't help.
Also, someone told me to use this make_ext4fs /dev/block/mmcblk0p15 command in the twrp command prompt. Does that have something to do with it?

Thanks for your help!