An Electron Cyclotron Resonance (ECR) magnetic nozzle plasma thruster typically consists of a microwave antenna and a magnetic nozzle. The magnetic field forms a region where the propellant is resonantly heated and ionised by the microwaves. The ionised gas is then accelerated out of the magnetic nozzle, generating thrust. The design of these thrusters to date has not accounted for the thickness of the resonance region, often assuming it to be near zero. Two test campaigns have been conducted in which the thickness of the resonance region was varied. Larger resonance region thicknesses are seen to increase thrust and specific impulse by up to 60 % and thrust efficiency by up to 32 %. Experimental measurements of electron temperature, plasma potential and ion current indicate that increased electron heating in the larger resonance volume leads to a stronger electrostatic field and higher ion beam current. This study presents strong evidence that optimising for a thickened resonance region can significantly enhance thruster performance.