I have the Martin Logan Ascent I and use the Theater I for centre. I use a front projector so that size isn't as much of an issue, as it doesn't have to be on top of a set.
I agree that matching timber is key, but I would suggest that a superb Center provides a much more material impact.
I was astonished by the original ML center, the Logos. I upgraded to the Theater I, and was even more impressed. I then upgraded the fronts from ML Aerius Is to the ML Ascents!
I didn't realize how much information came from the center until the theater I and having a dedicated power amp.
You might find like me, that the center channel may actually drive your choice in the rest of your system!
Best of luck