The value of magnetic field due to a small element of current carrying conductor at a distance r and lying on the plane perpendicular to the element of the conductor is?

The Biot Savart law is used for computing the resultant magnetic field B at position r generated by a steady current I.  

 $B=\frac{{\mu }_{0}I}{4\pi }\frac{dI\sin \theta }{r^2}$

sin($\pi /2$) =1 which is the maximum value.  
So magnetic field at a distance of r and lying on the plane perpendicular ($\pi /2$)to the element of a conductor is maximum.