Diameter = the length across the circle (the diameter of your 9" plate is 9 inches) Radius (r) will be half that: 4.5"
Circumference = the measurement around the edge of the circle (where you're trying to sew the bag together) is 2 times 3.14 times the radius (2*Pi*r), or 9*3.14 (if you're using the plate example).
So the length of the side to sew to the round part should be about 28 1/4 inches, plus seam allowance. If you're using the plate example.
If it's another size, I reccomend folding the circle in half, measuring that long flat part, and x by 3.14 to get your measurement (plus seam allowance).
Ha! I defy *anyone* to tell me HS geometry is a waste! LOL