Well, the Opti RGB would probably be closer to a 300w Par 56 lamp. As for beam sizes of Par 64 lamps, they are roughly as follows:
VN: 12 Degree
N: 14 Degree
M: 24 Degree
W: 50 Degree
Keep in mind they are rough numbers. Also remember that Medium and Wide lenses spread in an oval, not a circle. Here is a Cheat Chart for Par 64s. Granted, it is for 1000w, but the dimensions are the same for a 500w lamp.