Normally developers do not have Photoshop and aren’t design guru and, in general, certainly hate to redo their work to meet the needs of the expert designers.
So we decided to report a infographic guide designed for designers who want to make App for IOS and help send the correct files for the developer.

The infographic style explain everything with an image, we propose that vo below:

 


Source

First of all, as we see in the image needs to remember the size of different device from Apple.

Iphone and ipod touch until the third generation: 320 x 480 px / 163 PPI
Iphone4 and ipod tpuch generation: 640 x 960 px / 326 PPI
iPad & iPad2: 1024 x 768 px / 132 PPI

iPad 3: 2048 x 1536 px / 264 PPI

Apple recommends into using icons:

44×44 px
for control buttons

 

Another suggestion, you have to prepare the icons starting from size 512 x 512 px and 1024×1024 px in case of iPad with retinal display, always paying attention to visonare design when they are so much smaller.

The rest of the tips are much more clear if you read carefully what is reported in the picture posted above.

Source: infographic post, that all summed up by an image inserted in the FSM blog (http://www.funkyspacemonkey.com/ios-app-designers-guide-infographic)