How do I choose the right size?

I had a question. I know that when you start a new project is asks you if you want a blank presentation, or SD 4:3 or 16:9 or HD. What is the best size aimed towards Android? Or do you start with blank and set your own layout size? Also the dotted outline what exactly does that represent because it seems that most the backgrounds extend pass that border anyway. Thanks for the help.

I don't have an answer to your first question. There is though this tutorial about supporting multiple screen sizes.

https://www.scirra.com/tutorials/73/sup ... zes/page-1

As for your second question. The dotted line represents your window size, or in other words it represents the area of your layout that can be seen while your game is running.