What is a reasonable estimate for the radius of an atom?

Answer is: 1 × 10⁻¹⁰ meter or 0.0000000001 meter.

The atomic radius is a measure of the size of the atom.

It is distance from the center of the nucleus of atom to the boundary of electron cloud.

The radius of atoms is between 30 and 300 pm (10⁻¹² m)  or between 0.3 and 3 angstroms (10⁻¹⁰ m). 

The radius generally decrease along each period of the Periodic table.