The guide for Windows Phone 8 app bars states that AppBar button icons should be 48x48px. It also says that a bunch of sample icons can be found in C:\Program Files (x86)\Microsoft SDKs\Windows Phone\v8.0\Icons.
But the images in there are 76x76px. And they work fine, at least on the emulator they do.
So what’s the proper size?
76×76, because then you support higher resolutions. If you use 48×48, it’ll be scaled up for WXGA and 720p, rather than down (for WVGA), with the quality being slightly worse.
The factor is roughly 1.6, which is the same as for the Tile Sizes
And to prove the point, here’s our application, using 76×76 application icons: Skype | Windows Phone Store