I don't know if there is some variance between models/years, but in our '91 190E 2.3, the 'drop-in' size for the rear deck is 4". I note that some have installed a larger size, with mods and a non-stock grille.

Boston Acoustics makes a 4" model that is drop-in, and it also comes with break-away tabs and clips that make it drop right in the dash, as well.

I considered the Sony, since it is available in a similar size, but found the sound disagreeable compared to the BA's.

Steve