We have studied the magnetoresistance of a quantum point contact fabricated on a high mobility two-dimensional electron gas (2DEG) exposed to microwave irradiation. The resistance reveals giant B-periodic oscillations with a relative amplitude ΔR/R of up to 700% resulting from the propagation and interference of the edge magnetoplasmons (EMPs) in the sample. This giant photoconductance is attributed to the considerably large local electron density modulation in the vicinity of the point contact. We have also analyzed the oscillation periods ΔB of the resistance oscillations and, comparing the data with the EMP theory, extracted the EMP interference length L. We have found that the length L substantially exceeds the distance between the contact leads, and rather corresponds to the distance between metallic contact pads measured along the edge of the 2DEG. This resolves existing controversy in the literature and should help to properly design highly sensitive microwave and terahertz spectrometers based on the discussed effect.