Bobbins can be complicated and depending on what kind of machine you have, you must buy the correct size.
There are three basic "classes" of bobbins: class M, class L and class 15.
It is easy to differentiate Class L from the other two. Class L has a smaller diameter than class 15 and M. If class M or 15 is too big to fit into your machine, then you can be pretty sure that your machine takes only class L bobbins.
The difference between class 15 and class M is a little tougher to find.
First, determine if your machine has a vertical front-loading or horizontal drop-in hook. Once this is determined, you can find your correct bobbin at just about any sewing store.
The bobbins for vertical hooks are usually wider and can hold more thread. The drop-in style bobbins are usually not as wide and some have a slight dome on the top and bottom of the bobbin. In my experience, the dome isn't as important as the width of the bobbin.
Of course the easiest way to determine your correct bobbin size is to find one of the bobbins that initially came with your machine and compare the width and diameter with the new bobbins you've purchased. If they match, then you should be fine.
I have several different sewing machines and this is my method for determining which bobbin goes with which machine. Hope it helps!
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