OK , I have a 98 GXE and have replaced all four of my speakers. This is what I found.
The real speaker size is 6 3/4. However, 6 1/2 will fit if the speaker has the oversize ring around it. In other words the speaker has a ring with holes and notches in it that will cause it to fit in a variety of speakers. I my case I actually has to Dremel the holes in the oversize right to get the speaks to fit right.
An easy way to solve your problem would be to take the right passenger door panel off to access the stock speakers. You will find the stock speakers are mounted on a plastic bracket of some sort. Remove the speaker and remove the bracket. Take the bracket with you when choosing speakers to eliminate fit problems. Ask for the speaker sample in the store and try to mount it on to the bracket.
NOTE: In my install I preferred to use the original Nissan screws. You should probably try for this to.
Jay