I have created an application in which I have used an .xib for layout. Currently I use two different .xibs, one for iPhone4 320*480 and one for iPhone5 320*568. I am not using auto layout.
Now iPhone6 and iPhone6+ are available, with two different sizes, 4.7 inch and 5.5 inch.
How can I make my app compatible with these sizes?
Easiest way is to download Xcode 6 GM (which was released today) and then run your app and see how it looks. You may not have to do anything at all as iPhone 6 and iPhone 6 Plus will scale most older apps seamlessly. Reference: http://www.imore.com/all-13-million-ios-apps-will-instantly-scale-iphone-6-and-iphone-6-plus
I also tried this out myself on my app (designed for 4" screens) and it worked well in both the iPhone 6 and iPhone 6 Plus in the simulator.