Hello there @Stirliab!
A good rule of thumb is to get a compression bag that is the same dimensions as the stuff sack that comes with your sleeping bag. While you could probably go a little smaller than this, if you match the size of the stuff sack, you will have a lot easier of a time getting your bag in and can then cinch it down pretty small from there!
In the case of the Joule, it comes with a stuff sack that is 8"x18" This would translate into a 14L compression sack from Sea to Summit.
Hope this helps!
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