What is the size of an atom

Atomic radius is anything from 25 to 300 pm, depending on the atom. Of course, it's hard to define, as the electrons form a cloud around the nucleus of varying radius.

Here are some references with examples:

Size of the Atom - Triumf (http://trshare.triumf.ca/~safety/EHS/rpt/rpt_1/node7.html)

Size of Atom - Fairfield (http://www.fairfield.k12.ct.us/tomlinson/element/size_of_atom.htm)

Size of an atom - BU (http://www.bu.edu/core/cc105/lectures/L10-Atoms/howbig.html)