$\text{From Ampere's Circuital law inside the wire}$

$\int B.dl = \mu_0 I_{enclosed} = \mu_0 \frac{ I\times \pi r^2}{\pi R^2} $

$\Rightarrow B.2\pi r = \mu_0 \frac{ I\times \pi r^2}{\pi R^2} $

$\Rightarrow B = \frac{\mu_0 I r}{2\pi R^2}$

$\Rightarrow B \propto r $

$\text{Value of B at 1mm is }\frac{B}{2}$