What shape can be drawn through a solenoid to determine the magnitude of its magnetic field?

Amperes's law, one of Maxwell's equations, can be used to calculate the magnitude of the magnetic field in different situations. The law states that the integral of B (magnetic field) multiplied by a vector l is equal to the permeability of free space multiplied by the current. In order to use the law, the vector l needs to be drawn about the geometry. For a a solenoid, due to the symmetry, it would make sense to draw a rectangle that has one long side going through the solenoid and the second long side outside of the solenoid. 

 Rectangular shape

Explanation: we consider a rectangular shape when we find the magnitude of the magnetic field.

We apply Ampere's circuital law to a rectangular loop.

According to Ampere's circuital law, the magnetic field of a solenoid is directly proportional to the product of the current which is flowing around the solenoid and number of turns per unit line of the solenoid.