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In coupes they're 6.5", and can be replaced without pulling out the trim pieces, just pop the grill off by pushing up on the bottom edge then pullig out/down when the tabs release.
In verts, FRC, & ZO6 they're 5.25", and I believe you have to take out the trim to get at the speakers.
Any 2 or 3-way 'standard' 4 ohm speaker would be better than the stock paper speakers. The stock system probably tops out at about 15-20w RMS.
You can buy adapters to connect the new speakers, or easily just cut and splice the stock wires to what comes with the new speakers. Polarity is shown on this schematic.