What are the dimensions of this item?

It is 2.22 x 1.02 x 0.46 inches.

Whether if the 518 USB-C to DisplayPort adapter could support the display of another different screen?

No, it could only mirror your laptop's screen to a larger display or extend your desktop to use multiple programs at the same time.

Does it need any driver or installation accessory to use?

No, all you have to do is plug it in.

What is the difference between the 518 USB C to DisplayPort Adapter and the 518 USB C to HDMI Adapter?

The difference is the type model. One is a DP(Displayport) port, one is an HDMI port, please refer to the following picture.

Please contact us for assistance if you have any further questions.