As mp3tag cannot compare 2 files I doubt that there is an automatic way to find albums/files with varying cover sizes.
What you can do, though, is add a column in the files list that shows the cover size.
If you sort by this size but have gaps in the track numbering that shouldn't be there, then this might be a hint that cover sizes are not the same.
If you filter by artist you get a more handy chunk of data.
On the other hand: if you have the cover file already in the corresponding folder then you could import that file as cover art as version 2.49 supports wildcards and imports the first picture file found.
This would then overwrite the embedded cover art but all files in the folder would have covers of the same size.