The exact measurement of the front dash speaker is 119-120mm, just a bit over 4 5/8". In addition to MB factory snap in speakers and the ones scott mentions, Blaupunkt also makes an exact replacement in this size if you want a snap in. Senior forum member (and excellent tech advisor) Stevebfl offers them out of his shop at contenental imports:
http://www.continentalimports.com/ic_114.html .
Or if you want to go more towards higher end, you can install a wide variety of 4 inch or 5 1/4" speakers (not all 5 1/4" speakers are that size, some are larger diameter, some smaller.) Diamond Audio makes a 4 1/2" speaker that could fit - make a thin metal ring of 5" outer diameter, 4 1/4" inner diameter, mount the speakers to the ring, and the ring to the dash. The Diamond Audios can also be bi-amped by switching a jumper if you are replacing your head unit at some point - the stock unit only has 2 amp outs. Some members have also reported good results with DEI (Directed Electronics Audio - they just bought a/d/s high end speaker co!).
Some of the smaller diameter 5 1/4" speakers have been made to fit by others on this forum from Boston Acoustics and MB Quart, among others. These usually require more amplification than the stock head unit can provide. The main constraints are the cut out hole in the dash, the depth behind the factory speaker grill, and the area between the grill mounts.
If you decide to drill the holes to mount the speakers, a flex extension for a Dremel tool to a drill bit will work, but I have had good luck with the adhesive rubber mounting I described above. Just remember to check the depth, height, and diameters of the speaker you are choosing. Check the archives for more brands and ideas as well.