First I would like to know if anybody here generally have any preference with embedded art cover?
Is there an actual size standard? If so any reasons why?
I generally don't prefer to have album art bigger than 1000 x 1000, because it slows down MP3tag and anything that reads MP3 via file manager. 
So I was thinking it would make sense to have the ability to auto force any image to automatically scale to given size and have the option to only force those that are bigger than a fixed set resolution.
Have a look at this discussion:
Resize cover art
NOT MP3 Image > Save As.... + auto resize
BUT Import > MP3 Image + auto resize
The way i'm suggesting it is the other way around though. You know. Vice Versa?
Well, then you could supply files that have the correct size already.
AFAIK has IRFANVEIW a batch function to resize pictures.
And, I guess, any dedicated graphics program is better in handling graphics than MP3tag will be.
OK if that's the case then tools are made specifically for what i'm asking for.
Embedding those images as really large sizes like 1920 x 1080 won't hurt at all as a standard issue to size & lastly performance-wise?
An external Tool is no more necessary.
Mp3tag can resize cover art itself.
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Action: Adjust Cover – Mp3tag Documentation
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GUI: (Right mouse click on a existing cover)
Available since Mp3tag v3.01 from March 2020:
Remark:
This is not an "auto force" resizing for every added cover art!
You have to call the resize Action or the GUI manually and define the size you want for your covers in the selected tracks.
