One measure of size is the element-element distance within the element. It is not always easy to make sensible comparisons between the elements however as some bonds are quite short because of multiple bonding (for instance the O=O distance in O2 is short because of the the double bond connecting the two atoms. The bond length in CmCm is: 347.7pm.
There are several other ways ways to define radius for atoms and ions. Follow the appropriate hyperlinks for literature references and definitions of each type of radius. All values of radii are given in picometres (pm). Conversion factors are: