Hello NIC
Your 123 uses 4" for sure up front. I looked at the hole in the back and really doubted that 5 1/4 would fit...I doubted that 5" would have fit, so I bought 4" for the rear too. For the most part you are correct in assuming that just about any brand would fit...in the rear anyway...up front, you will most likely have to cut off a small piece (of the metal speaker surround) off of the corner, so it will fit in its place. Also do not get anything too tall, it will push up on the factory grilles. (not a problem in the rear). I put 3-ways up front, and 2-ways in back, although you may want to opt for something with a bit more bass in the rear. Once you remove the originals, you will see a sorry excuse for a speaker, as this was obviously before MB discovered that there was money to be made in decent sound, not to mention demand for it.
Good Luck, and have fun!

PS: Are you replacing the factory radio too?

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It is a truism that almost any sect, cult, or religion will legislate its creed into law if it acquires the political power to do so. Robert A. Heinlein


09 Jetta TDI
1985 300D