Hi,
There is no limit for Album art (I had some FLAC files with 35MB images embed that is almost size of audio itself), but it is possible that picture format is not read by Google Media Scanner and MMA do not show it.

If you find one file that is constantly not showing Album art, may I suggest taht you open support ticket and supply us with download link so that we can analyze and test why it is happening.

I have been going through my collection and updating, as we all do, and I have been making sure I have artwork embedded into the files. I am using Mp3tag v3.11. I have been pulling my artwork from albumartexchange.com and I have been grabbing the largest and best looking of what is available. Usually a 1500x1500 and whatever looks "cleanest". Some of these files have been huge (as much as 4MB), but I wasn't overly concerned with it. I am running into an issue where some artwork won't show up on MMP on my Android phone. But it will show up if I use AIMP on the same phone. It's not all albums, just some. and it's always the whole album won't show artwork, not just some files. It is not a pixel size issue, it seems to be the actual size of the artwork jpg that gets embedded. I don't know that I need 1500x1500 pixel artwork, but I figured why not.

I think I have discovered that the root issue is a 2MB file limit to the artwork before MMP won't show it. I replaced the artwork with smaller file-size ones, and they work. I am sure this is my fix, so I'm not really looking for assistance, more just sharing my experience in case someone else runs into the same issue. I think from here on out I will limit it to 1000x1000.