I have thought of testing this out on my Lizzie but not got around to it yet. You can only try one on a practice sandwich, (manually only) but will the hole in the smaller bobbin be the same as the M bobbin? If the hole is larger you will have a lot of backlash. I know the M bobbin will not fit on the winder spindle of my DSM - I have tried that.
Don't understand why the company would send L size to you if they weren't sure they'd be ok.