Real IPD is the actual distance between the center of your eyes' pupils in real life. Any Head-mounted Display require this measurement to accurately position the virtual camera outputs on the screen in front of the user's eyes. The outputs have to be directly in front of both of the users' eyes. Any deviation can cause eye strain. Most HMDs allow users to adjust their camera output positions to match their IPD.
The average IPD for humans is 64 millimeter, with a range of 54 to 72 millimeter. You can measure your IPD by measuring the distance between the center of your pupils with a ruler.
Developers can change the sense of scale by changing virtual IPD. They can cause the environment to increase or decrease in size by changing the virtual IPD.