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They are 4"x6" speakers. If you've not worked on this dash set-up before, consider the following issues:
The speakers ride in metal housings which screw up to the bottom of the dash. These keep the speakers from rattling and are a "good thing" to reuse with your new speakers. You may have to use a pencil-type soldering iron to put mounting holes in the correct places on the new speaker frames to re-use these housings.
Someone else on the forum suggested putting pieces of pantyhose material over the new speakers to keep "stuff" from falling into them [thru the dash holes] and preventing rattling. Good idea!
The old dash pad is not so easy to remove; it can crack if stressed too much. I changed mine by just removing the rt/lft dash pad screws, the upper mounting bracket screw (on the right side), and pulling the dash loose from the attaching clips. You can then lift it enough to do your business with the speakers.
You might want to consider putting 5"x7" speakers in the lower kickpanel areas, also. If you do, I'd suggest that you hook the speakers up in series (each side) [rather than in parallel]. You may lose a bit in total volume output, but you'll not overpower your amp and the sound might be a bit cleaner.