Give a method to measure size …

By Avogadro's hypothesis, the actual volume occupied by the atoms in one gram of a substance is 2/3rd of the volume occupied by 1 gram of the substance.

Let us consider a sample of a substance of mass m and volume V. If M is its molecular weight and Avogadro number is N, then

Number of atoms in the given sample = ${N\over M}× m = {Nm\over M}$

let each atom is a sphere of radius r..

So 

Actual volume of the atoms in the given sample = ${Nm\over M}×{4\over 3}\pi r^3$

 By Avogadro's hypothesis

${Nm\over M}×{4\over 3}\pi r^3 = {2\over 3}V$

If $\rho$ is density of substance . Then

$\rho = {m\over V} \\=> V = {m\over \rho}$

$=> {Nm\over M}×{4\over 3}\pi r^3 = {2\over 3}{m\over \rho}$

$r = \left ({M\over 2 \pi N\rho }\right )^{1\over 3}$

Knowing values of M,N and $\rho$

We can find radius