How to know the hard disk capacity for sure and how to recover lost partitions?
Apple exposes the hardware size of the drive in several places.


About this Mac in the Apple Menu - click storage tab
System Information app (click System Report... in About this Mac - overview section)


there's a storage general category 
there's specific storage details (firmware / bus position / bus speed) for the SATA / AHCI / NVMe section based on how your storage is physically connected to the hardware

Disk Utility (in /Applications/Utilities)


The undo is either finesse the partition table (there are hundreds of questions here on how to change gpt and diskutil or to backup your files, and erase install a new OS which basically boots to another recovery OS and runs Disk Utility to repartition / erase the drive.

Good luck with your choice of path to get the storage back and set.