How to create new emulator with custom screen size?

If you are using the latest version of AVD Manager, you will find a new tab labelled ** Device Definition**.

Here select new Device, and then specify your Screen Size(in) and Resolution(px). You can also specify Size, Screen Ratio and Density besides other things.

Open the Android Virtual Device Manger go to Device Definition tab, create a new device.Then, Create a new emulator and choose your device from the drop down menu.