I designed my app using a 320x568 Layout (iPhone 5)
When I run it on an iPhone 6 there are two margins on the bottom and right (the remaining of the bigger screen)
I would like to know if it´s possible to STRETCH the whole layout to cover the whole screen depending on the device screen size.
I don´t want to use any anchors because my app does an intensive use of canvas draw functions.
The standard solution is to use anchors and the designer script for this. In your case you will need to implement a scaling feature that will scale your drawing.
I'm not sure that it is the best idea to build the interface based on canvas.