Well the layout size doesn't really matters here. What really matters is the window size,backround size (to cover gaps) and the anchor behaviors.(to keep everything relative to window)
The game in my example was a pinball like game so it was very important to keep all the stuff in place (the game surface is bigger in age of zombies so relative position variation inside of the layout may be less important.
The first question you will ask yourself is what kind of mobile platform do you aim?
If you want your game to work from iphone 4 to ipad then you must support the 3:2,4:3,16:9.
Which means (i give an example) that your window will be: 1152*768 (3:2) and layout will be 1366*768. This kind of configuration is good for pinbal games but it will maybe seem strange and not in proportion for other games when you will test it on 16:9 ratio. So you should maybe pick another layout or windows resolution from the link i have put above.
It's all about proportions.
I advise you to make a basic setup and give it a try.