A full size spool fits nicely inside... and the thread goes up and through the carrying handle of my Travel 153 Bernina.... then on into the normal thread feed of the machine. Why do this? Sometimes these large spools ( especially when full/new) like to tangle at the base of the machines spool holder ( this is my only complaint about this older Bernina). So... I sit the 'napkin ring with spool a distance away from the back of my machine.. and thread as noted. It works quite well.
Indeed, for quilters, where there's a will... there IS a way!!!
The next tip is one I'd guess all quilters that regularly use monofilament know. There are tricks to filling a bobbin ( I almost exclusively use mono in my art quilt bobbins.)
1) It's pretty simple.. wind it slower than pedal to the metal. Stretching the mono with a high speed winding stretches it and makes it a bit more apt to break. Be light on the foot pedal or slow your machine speed for those with no foot control on bobbin winding.
2) Fill the bobbin only 2/3 to 3/4 full. This thread is around 5 times thinner than an average weight piecing or quilting thread. You don't need a full bobbin AND a full bobbin is more difficult to get into your bobbin case. I want to see some daylight between the threaded bobbin and the engaged bobbin winder lever. This one is as full as you would ever want
Happy quilting all.