Is it that electron of an atom can be found anywhere in the space? 
Simple pictures showing orbital shapes are intended to describe the
  angular forms of regions in space where the electrons occupying the
  orbital are likely to be found. The diagrams cannot, however, show the
  entire region where an electron can be found, since according to
  quantum mechanics there is a non-zero probability of finding the
  electron anywhere in space. Instead the diagrams are approximate
  representations of boundary or contour surfaces where the probability
  density | ψ(r, θ, φ) |2 has a constant value, chosen so that there is a
  certain probability (for example 90%) of finding the electron within the
  contour. Although | ψ |2 as the square of an absolute value is
  everywhere non-negative, the sign of the wave function ψ(r, θ, φ) is
  often indicated in each subregion of the orbital picture.

  [Cross-section of computed hydrogen atom orbital
  (ψ(r, θ, φ)2) for the 6s (n = 6, ℓ = 0, m = 0)
  orbital.]     

I have a question here, if electrons can be found anywhere in the space with non-zero probability, can we give a definite boundary for the atom? i.e can we determine the radius of atom?   
My sir has said me that, radius of atom is around $10^{-10}$m (from the $X$-ray experiments), but as we can have non-zero probability of finding the electron even beyond $10^{-10}$m, how can we say specific radius of an atom?
 A: You are quite correct that atoms don't have a precise size. When defining the size of atoms we tend to use either bond lengths, if the atoms are reactive, or interatomic potentials for atoms that aren't reactive.
For example take Argon atoms, which are unreactive. The force between two argon atoms is well described by the London dispersion force, which in the case of Argon looks like:

Typically we get a minimum in the energy (at about 380pm in this case) then a hard core repulsion i.e. the energy rises steeply as you push the atoms past the minimum. We can take this as a measure of the size of the atom.
Where atoms react we can use the spacing in the molecules as a guide. For example the O to O distance in the O$_2$ molecule is 121pm giving us a radius for the Oxygen atom of a bit over 60pm.
However we will get different values for the atomic radii depending on how exactly we define it. For example Carbon forms single, double and triple C-C bonds, and they all have different lengths giving us different radii for the Carbon atom. This means the figures for the sizes of atoms are a guide rather than a precise value.
Wikipedia has a list of atomic sizes here that shows the different values for the radii obtained using different measures.
