Hi Rebecca,
Thanks for reaching out! that kind of depends on what you want the finished product to look like. If you are hoping for a smaller sized thread then go with size 8. If you want a slightly bigger thread then I would recommend size 5.
At least 2 places that I see online seem to recommend size 8 though. Here is a link to an example of what the finished product might look like if you use size 8 thread. Thanks again for reaching out! Feel free to let us know if you have any other questions and have a great day!
Leah W.
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Rebecca H.
What size would you use to tie a quilt?