They are the same size as the dash speakers. At least they were on my wagon when I replaced the Boston Acoustics that were in there with Kappa Infinities. Look out though - newer speakers have bigger magnets and that door well does not take well to customizing. I had a devil of a time getting the speaker grills back on.
Edit: just realized that wasn't a very helpful post so I looked at the old ones which were FX-4s, 4 inch speakers. Here's what I put in the back doors:
I have used Blaupunkt speakers with great success in all my W124s. If I remember, the rear door were a drop in replacement, making it plug & play. I think mine are 4" units. All I had to do was swap over the wiring.
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