To determine the length of a solenoid in a magnetic field, a specific formula is referenced: length = 2*T*R*N, where T is approximately 22/7, R is the radius, and N is the number of loops. The discussion seeks validation of this formula and additional methods to find the solenoid's length when other parameters are provided. The magnetic field inside an infinitely long solenoid is given by B = 4 π x 10^-7 nI, where n is the turns per meter and I is the current in amps. Participants express a need for a straightforward rule to calculate the length when other variables are known. The conversation highlights the complexities of solenoid calculations in practical scenarios.