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?
So what's the proper size?
76x76, because then you support higher resolutions. If you use 48x48, 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 76x76 application icons: Skype | Windows Phone Store
They are 48x48px as the people above have said, but the 76x76px ones you are seeing are for devices with a different screen size from that set by Visual Studio as default.
If you are looking for any additional icons do check out Templarian's open-sourced icon project http://modernuiicons.com/. The .zip includes light and dark themes icons as well as the initial Expressions Blend .design file and the .xaml coding of the icon.
They have to be a minimum of 48x48, you can have a bigger (square) size and it will scale down accordingly.
来源：https://stackoverflow.com/questions/13323383/winphone-8-appbar-icon-size