Reply by DS

Wow, lots of numbers flying around here. You can get the sense that there is a wide range of standards and options.

From the furniture side, standards vary a bit. Typically 20”deep and 30” high for base cabinets in this situation.
You want to be able to mount your TV at a good height in the room and still have a nice field around it to your cabinets. 30”h typically works fairly well for this.

Kitchen counter height is good for working on a surface, but is a little too high for an entertainment center situation.

20” deep is about the minimum depth to fit an old style cable box with rear connectors in it. (about 19” clear inside)
A modern box could be smaller and not need so much depth.

Be sure to plan for your wire access throughout the unit. It’s much harder to create access after installation, than plan access holes in advance.

Plan where all your equipment and speakers will go and ensure you have proper room for each item. After that is done, the rest of the design is pure aesthetics.

Have fun!

-- "Hard work is not defined by the difficulty of the task as much as a person's desire to perform it.", DS251