In a single slit diffraction of light of wavelength λ by a slit of width e, the size of the central maximum on a screen at a distance b is

$\frac{2 b \lambda}{e}+e$

The direction in which the first minima occurs is $\theta$ (say).

Then $e \sin \theta=\lambda$ or $e \theta=\lambda$ or, $\theta=\frac{\lambda}{e}(∵ \theta=\sin \theta$ when $\theta$ small)

Width of the central maximum $=2 b \theta + e=2 b . \frac{\lambda}{e}+e$