Another way to think about this: how many blades/petals do you wish your completed block to have? Once you decide on this, just about all the work is done, really, because the number of blades/petals determines the width at the center of the block ... and the diameter of the block determines the width at the top of the blade/petal, and in the end determines the final diameter of the center circle of the block.
Pick a ready-made block with the number of blades/petals you want and print one out. (The 12-blade block pattern is tough to come by for some reason, they're "all" seemingly 8 or 16 blade blocks). Tape or glue this ready-made block onto a larger piece/sheet of paper or template material and use a ruler or straight edge to extend each of the blade/petal lines outward until your eye says "STOP." This will pretty much be your final block size. Determine the final blade length and then decide whether to make it a rounded or pointed blade/petal. Then audition different center circle diameters to ensure that the center circle is proportionate - or pleasing to YOUR eye - to the blades/petals. You could even have some fun fussy cutting the circle fabric, embroidering or appliquing something special in the center of each circle.
I hope my suggestion was helpful to you and not too confusing - PM me if you have any questions!