Like Drew said, creating a user defined device definition has been the only solution which worked for me. Below i show you the steps to follow:
Step 1)
Within the preview-pane, open the Virtual Device Selection drop-down and select Add Device Definition..
Step 2)
Within the Your Virtual Devices dialog click the Create Virtual Device button.
Step 3)
Within the Select Hardware dialog click the New Hardware Profile button.
Step 4)
Within the Configure Hardware Profile dialog specify (f.e.) a Resolution of 720 x 4000 px and a Screensize of 12 inch. Also setting the Screensize results in a density of xhdpi (thanks to Mete).
Step 5) Close all dialogs and restart Android Studio.
Step 6) Open the Virtual Device Selection drop-down. The new user defined hardware profile can be found under Generic Phones and Tablets.
