According to Amazon Kindle Publishing Guidelines
it depends on book format:
3.5.2 Image Guideline #2: KindleGen Performs Automatic Image Conversions
The limitation on the image size depends upon the book’s format:
- Reflowable books (fiction and non-fiction): 127 KB maximum
- Fixed-layout books (other than comics): 256 KB maximum
- Comic books: 800 KB maximum
But then they also recommend saving in 300dpi at a minimum size of 600x800.... who knows??
I haven't seen anything different for .azw3 ???