How do I find my frame size?

I need to replace my AC compressor fan motor, but I can't find the exact model number. I know if I call and give Grainger the specs, they can help find an acceptable replacement. The only spec they're asking for that I don't have is the frame size.

Is the frame size a property of the motor itself or of the outdoor AC unit itself? There is a place on the AC unit where a name plate used to be, maybe the frame size was listed on that.

Is there a way I can take a measurement or something? Any advice will be greatly appreciated. It's friggen hot here.

I can't find what I need on Google.