What KalessinDB said is correct.
However, they made a few special Sega CD's that did not require a Genesis because the Genesis was built in. The CDX will be the smallest unit that has the built in Genesis and CD, but it's super expensive if you're lucky enough to even find one.
Do you not have space to go wider or taller? If you get a model 1 Sega CD, it goes underneath and therefore it isn't much wider but it will be taller. If you get a model 2 Sega CD, it goes next to the Genesis, so it's a lot wider, but not very much taller.