When I eventually did was enter the following command into the terminal app inside of TWRP:
mke2fs -t ext4 -L data /dev/block/mmcblk0p26 11901935
*PLEASE NOTE: The 11901935 number is specific to the 16GB Sprint GS5. If you have a 32GB version you will want a different number.
*ADDITIONAL NOTE: Although the system runs I am still unable to encrypt the device.
partition table for g900p:
PS: Some posts on this subject suggest that the "11901935" value might need to be 16 fiddled with a little. If you are unable to encrypt your phone afterwards you might need to re-run the above command and subtract 16 or so from the above number.