I put in a back counter and it is 12" wide. I wanted it as narrow as possible to utilize the space behind my theater chairs.
The 12" width is more than adequate for me. There is plenty of room to rest your arms on it, or place your drink or a big container of popcorn. I ate at it during the super bowl and it was fine for that use as well.
If you can go wider, that would be great. But for functionality, 12" wide is sufficient. The cabinet the top sits on is a U shape, so you have plenty of room to slide underneath the cabinet as well. I do have four feet behind the structure to the back wall.
My back surrounds are bipoles, so the sound while not as good as the chairs, is still good enough. the side surrounds are direct so that hasn't been compromised much either.