How to resize the main volume to its original or full size?
You are not required to use core storage unless you have a Fusion drive or need encryption. In your case, the drive does not have any core storage partitions. Your question does not contain enough information to determine what hidden partitions may exist on your drive. To determine this you would need to edit your question and add the output from the following commands.

diskutil list
sudo gpt -r show /dev/disk0
sudo fdisk /dev/disk0


You should enter these commands while booted normally to OS X. Some commands may ask for your login password. This is normal.

Assuming you just have a EFI partition closely preceding and a Recovery HD partition closely succeeding the Mackintosh HD partition, you can add the blank space to this partition by following these steps.


Boot normally to OS X.
Setup the Disk Utility application just as shown in your image.
Enter a new size of 1000 GB, then press the return key.
The size should change to the maximum value.
Click the Apply button. Note: This can take a while if there is a
hidden Recovery HD partition.