I moved this to the Tableau Developer Community where it's more likely to receive a response.
Thank you for your answer.
Just to give more details, I'm building the app from html5/js and we use Adobe phonegap to generate the final code.
It's working if I put 2x smaller than the screen pixels size. But we would like the dashboard to adapt depending of the phone navigating the app. Even if I choose the dashboard size to "automatic" it dosen't resize automatically.
Thanks for your help.