size "M" is for longarms...size "L" is for regular machines. HOWEVER, not all machines can take them. Berninas, Husqvarna/Vikings, and Pfaff usually can not use any of the prewound bobbins.
Signature makes a nice cotton one, and Superior threads has the Alex Anderson Masterpiece threads...LOVE them both.
I have been using the Signature ones for about 10 years now. They fit my vintage singer, my Janome Jem platinum, and my Elna with no issues!