According to the id3 standard
" the tag . Maximum tag size is 256 megabytes and maximum frame size is 16 megabytes"
So I would say that the maximum size of a picture is 16 megabytes.
Any other restrictions then apply to particular players - and as there are so many, it is hardly possible to guarantee any size to be foolproof.
One still has to try it with every target system.
The only thing that can be said: MP3tag will support you to adapt the tags to the needs of the target system.