What is the range of sizes for molecular systems?
Well apportioned, mixed, and cured epoxy glue is a single covalent molecule.  Single crystal white diamond is a single covalently-bonded molecule.  On the small end, a dihydrogen molecule, is 0.7414 Å (ortho or para, your choice to trim the decimal).  

The largest human chromosome is ~220 million base pairs, or about 1.5x10^11 g/mol.