What should be the image/bitmap size in Android BigPictureStyle push notification?

You can find a Google I/O slideshow presentation here that mentions this (presentation by Chet Haase, Romain Guy & Dan Sandler).

Specifically on slide 52 it states about BigPictureStyle:

Protips

Images should be ≤ 450dp wide, ~2:1 aspect Bitmaps outside these bounds will just be wasting RAM (and possibly exceeding Binder IPC limits)

Note that this presentation is a couple years old, but there is no reason to deviate from its suggestions.

Related: https://code.google.com/p/android/issues/detail?id=36744