I'm developing the app to work both on iphone4 and iPhone 5. I'm using the following code to get the screen size
CGRect screenBounds = [[UIScreen mainScreen] bounds];

am using the iPhone 5 simulator but it showing the screen size of iPhone 4.
On the simulator make sure you have set to iPhone (Retina 4-inch). You can do this by going simulator menu at the top go to Hardware>>Device>>iPhone (Retina 4-inch) that will change the phone simulator to the iPhone 5 version.
If this is set then you may need to add a launch image for the iPhone 5. [email protected] if this is missing it will not launch in iPhone (Retina 4-inch).
I have also answer another question here that may help.
In fact
[[UIScreen mainScreen] bounds].size.height
returns 568 for iPhone 5. Make sure, you selected "iPhone 4 - (4 inch)" as Device in the Simulator (it's there since Xcode 4.5). There should also be an [email protected] file in your project if you created your project using the Xcode assistent.
(CGRect) screenBounds = origin=(x=0, y=0) size=(width=320, height=568)3,5" retina(CGRect) screenBounds = origin=(x=0, y=0) size=(width=320, height=480)- rckoenes[email protected]. Read the Apple documentation it will tell you you need the[email protected]in order to run in fullscreen mode on the 4" devices. - rckoenes[email protected]launch image. Also read the Apple documentation - rckoenes