first of all I believe the rear decks are 6 x 9 in the back and 6 1/2 in the doors and up front. The grill is not attached to the speaker it just has little plastic clips it should just pop off try using a screw driver. The speakers are attched from the top and bottom. You must unattach them from inside the trunk and then unscrew them from the inside of the car and they come out from the inside of the car. Make sure not to break off the connector clips for the gril because it will rattle like biatch if you do. If you are replacing the fronts good luck because you must take off the entire door panel to get to them the grill there now is fake it doesnt coem off!
If you have the Bose stereo, I understand you have to replace with Bose speakers (assuming you still have the OEM Bose stereo). I have the base model so this was not an issue and I bought off the shelf speakers. Hopefully some others with the Bose can shed light on this.
I believe that the bose system are 6x9 in the back because my 88 coupe speakers in the front doesn't look like the back and when i measured the back to see what size it came out around 6.9 inches kind of look like a oval egg shape.
I have an 88 coupe Legend and I had no problem re-installing some Kenwood speakers, all you have to do is put some elbow grease into it man and pop the covers.... then unscrew them with a cross-head, thats all I had to do!