How to measure how big rear speakers would fit to my Corolla?

My Toyota Corolla 1998 sedan doesn't have any rear speakers, so I was thinking of buying a pair.

How do I measure the size of the speakers that will fit the holes in my trunk? I took tape measure with me and went measuring and the diameter of the hole was about 11cm (4,3") and there are two little holes that would keep the speaker in place, I measured them from the outer side to the outer side through the speaker hole and it was 16,5cm (6,5"). Then I googled what speakers would fit my car and everywhere they are saying 6,75". I can't seem to find out how to measure so that I would get a result of 6,75". What is the correct way of measuring, how do I know that the speakers I would buy fits the holes in my car? I'm not really into making the holes larger so I'd really like to know the real size before buying anything that could then turn out to be too large or too small.

Also, where have they hid the wires for the speakers? I'm sure they have pulled the wires ready for speaker installation as I just recently replaced a radio and they were there. But I couldn't see any of the wires anywhere close to the speaker holes now that I was measuring the holes. I can see some wires going at the side of the trunk but I believe most of them are just for the lights. Perhaps the speaker wires are going in the same bunch. Where possibly are the ends of the wires?

I can provide images of my trunk if needed