Jags do not come in standard sizes and patches do not come in standard size or thickness. You need to find a combination of patches and jags that work for you for example by cutting your own patches.
You want to be able to push the patch through the barrel without too much effort.
I have a set of jags (a dozen or so) and I use the jag that works with my patches. I start with a small jag and increase the size until I have a small resistance when I push the patch through. You get get a set of jags for $10 at Cabelas and I am sure at many other places.
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