The sail area is not all that much on a Cal 21. (We raced and cruised a Ranger 20 for years...) With modern low stretch lines (yes, avoid Sta-setX) you should be fine with quarter inch. Heck, 3/16 would do fine, but it is a little harder on the hands.
While some sailors still use wire-to-rope lines, this technology was superseded in the 80's. Wire was hard on spars and sheaves, too.
Kind of like enjoying the look of traditional flax sails and the golden glow of the light from kerosine lanterns...
Example: by comparison, 3/8" is too large a diameter for the sheaves on my O-34 ! I have used 5/16" T-900 line for over 20 years. It will never wear out....
Figure our how much length you need, add 5 feet just-in-case, and order it up from whoever has a reasonable price.
One quick example found on line is $1.71/foot for the quarter inch. I bet that this site's store could offer you a good deal............