When using MTP (aka WMDM) to talk to the device, MC already caps the cover art at 128x128.
By "cap", do you mean it doesn't load it, or it resizes it?
The Sansa can be set for MTP or MSC. I've been using MTP all along. My library is in FLAC, but I transcode to WMA for loading the Sansa. Cover art is in the audio directory in the main library as folder.jpg. MC is also set to incorporate the cover art in the file (File Location > Cover Art > Also store art in ... tag). MC12 recognizes the Sansa in MTP mode and loads it up fine. To explore this problem, I switched the Sansa to MSC mode and browsed with explorer to see what was happening to the folder.jpg.
The audio files are in their respective directories as set up. There are no folder.jpg files. The cover art is all embedded in the file tags (which must happen during transcoding) and Sansa must be getting it from there (Sansa can get the art from either the file tag, or the directory).
The art in the tags, tho, are not 128x128 as you suggested. They are exactly the same as the folder.jpg file in the original library. I looked at a file where the art actually appeared in the Sansa display and it was 300x300 and 29KB. This is contrasted with a second file from another album that was 455x455 and 42 KB. Only the former cover art shows up in the Sansa display because of the 32KB limit I mentioned above. And obviously, neither are 128x128.
So is something not working correctly on the Sansa? ?
eCo