Resizing Image/Chat Bubble according to Text

I am making something like the WhatsApp Chat. When you send a message, a greenish rectanglepops up with a small arrow on the top right hand which resizes according to the amount of text you put in.

I would like to ask for help in doing the same using Xamarin Android (C#)

How do I resize a rectangle according to the text one inputs??

Somewhere on StackOverflow, I was told that Eclipse/Android Studio use something called 9 patches which allow them to resize things without padding or some stuff. I didn't really get it. Someone explain this to me with context to Xamarin.Android??

Thanks!!