Usually you set the dimension of a “virtual screen” that you will consider the real screen inside the game will be rendered.
This way you can easily project the dimensions of your actors ecc.
Then you use a ViewPort that scale your “virtual screen” to fill the real screen (that has its own real dimension in pixels).
Obviously there’s a problem about the aspect ration.
Typically it’s better to choose the dimensions of the “virtual screen” properly; for example if you choose a virtual ratio of 800×480 the game won’t be look stretched on devices that has the same aspect ratio of the screen.
This in not the only solution…maybe it’s better if you take a look here [https://github.com/libgdx/libgdx/wiki/Scene2d].