Large collection of common IOS controls

Element widget size (PTS)
Window (including status bar) 320x480
Height of Status Bar 20
Navigation Bar Height 44
Height of navigation bar with prompt 74
Navigation bar icon 20 × 20 (transparent PNG)
Tool Bar Height 44
Tool bar icon 20 × 20 (transparent PNG)
The height of the tab bar is 49.
Tab bar icon 30 × 30 (transparent PNG)
Vertical keyboard Height: 216 and 252 (Chinese Keyboard for iOS 5 +)
Average keyboard Height: 162 and 198 (Chinese Keyboard for iOS 5 +)
IPhone app icon:

Required icon name size and usage
Icon.png 57 × 57 App Store and iPhone, iPod Touch desktop display is
Icon@2x.png 114x114 icon.png's HD mode is
Icon-Small.png 29 × 29 "set" app and soptlight search no
Icon-Small@2x.png 58x58 Icon-Small.png HD mode No
IPad app icon:

Required icon name size and usage
Icon-72.png 72X72 iPad desktop display is
Icon-50.png 50 × 50 Spotlight search no
Icon-29.png 29*29 "set" Application No
IPhone and iPad General app icon:

Required icon name size and usage
Icon.png 57 × 57 App Store and iPhone, iPod Touch desktop display is
Icon-72.png 72X72 iPad desktop display is
Icon-50.png 50*50 iPad soptlight search no
Icon-29.png 29 × 29 "set" app and iPhone, iPod Touch soptlight search no

Extended reading: points vs. pixels

The iPhone 4 introduced a high resolution display with twice the pixels of previous iPhones. however you don't have to modify your code to support high-res displays; the coordinate system goes by points rather than pixels, And the dimensions in points
The screen and all UI elements remain the same.

IOS 4 supports High Resolution displays (like the iPhone 4 display) via the scale property on uiscreen, uiview, uiimage, and calayer classes. if the object is displaying high-res content, its scale property is set to 2.0. otherwise it defaults to 1.0.
All you need to do to support high-res displays is to provide @ 2x versions of the images in your project. see the checklist for updating to ios4 or apple documentation for supporting high resolution screens for more info.