Android icons (and other UI elements, like drag lengths) are measured in dp. A dp is a device/density-independent pixel. 1 dp is equivalent to 1 px on a 160 dpi screen. But to convert to other screen densities, you need to multiply it by a density factor. So it's generally recommended that multiple images are supplied for most icons.
For example, the notification icons used in the status bar are specified as 24x24 dp, with a 1 dp margin (so the actual icon only takes up a 22x22 dp optical square, though some of the AA can bleed into that 1 dp margin/safeframe). To convert 24 dp to actual pixel sizes, these rough calculations are used:
display density dp units * scale = px units
ldpi ~120 dpi 24x24 dp * .75 = 18x18 px
mdpi ~160 dpi 24x24 dp * 1.0 = 24x24 px
hdpi ~240 dpi 24x24 dp * 1.5 = 36x36 px
xhdpi ~320 dpi 24x24 dp * 2.0 = 48x48 px
xxhdpi ~480 dpi 24x24 dp * 3.0 = 72x72 px
There's also an intermediate display density called tvdpi (~213 dpi) that sits between mdpi and hdpi and has a scale factor of 1.33, but this is much less common. What the Android docs recommend is that you follow a 3:4:6:8:12 scaling ratio when providing prescaled bitmap images (usually PNGs) for the most common display densities.
I don't see anywhere where they specify the dp size for the large icons used in notifications, but the height of each notification in normal inbox view is 64 dp. So that means the max size for icons/images shown there would be:
ldpi: 48x48 px
mdpi: 64x64 px
hdpi: 96x96 px
xhdpi: 128x128 px
xxhpdi: 192x192 px
If you want to know exactly what image sizes Android's stock icons are, you should be able to find out from the Android Icon Templates Pack, v4.0.
See also this SO question: Honeycomb notifications: How to set largeIcon to the right size?
