According to my practice, I use the 40 x 40 for standard iPad tab bar item icon, 80 X 80 for retina.
From the Apple reference. https://developer.apple.com/library/ios/documentation/UserExperience/Conceptual/MobileHIG/BarIcons.html#//apple_ref/doc/uid/TP40006556-CH21-SW1
If you want to create a bar icon that looks like it's related to the iOS 7 icon family, use a very thin stroke to draw it. Specifically, a 2-pixel stroke (high resolution) works well for detailed icons and a 3-pixel stroke works well for less detailed icons.
Regardless of the icon’s visual style, create a toolbar or navigation bar icon in the following sizes:
About 44 x 44 pixels About 22 x 22 pixels (standard resolution) Regardless of the icon’s visual style, create a tab bar icon in the following sizes:
About 50 x 50 pixels (96 x 64 pixels maximum) About 25 x 25 pixels (48 x 32 pixels maximum) for standard resolution