What is the maximum image size in enhanced asset or broadcast feed?

As of early 2017, counterblock (which is used by Counterwallet) downloads PNG images of up to 20 kiB (20480 bytes) in size and 48x48 dimensions.

This is not a protocol limitation, but just a way one implementation does it.

Another wallet could impose no limit, or a different (smaller or larger) limit, but not that any user using a different wallet (such as Counterwallet) may not be able to see images larger than mentioned above.