Q. "How will I know which size compression fittings to use for my copper tubing?" 
A. You will need to measure the outside diameter of your copper tubing. Whatever the outside diameter of your copper tubing is will be the size of the compression fitting you will need. For instance, if the outside diameter of your copper tubing is 1/4" then the compression fitting will need to be 1/4". Compression fittings are not sized like copper sweat fittings. 

Q. "I have clear vinyl tubing, can I use these...

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