How to reformat a HDD to show the proper size?
The easiest way to fix this is to boot using your bootable USB, go into terminal and enter the following command to (re)partition your drive

diskutil partitionDisk /dev/diskX 1 GPT HFS+ MacintoshHD 100%


Where X is the number of the drive that corresponds to your HDD.  Issue the command diskutil list to get a listing of the disk identifiers.

Verify that your drive is showing up correctly by issuing the command

diskutil info diskX


Then proceed with your installation of macOS