How much walking space/clearance do I need when laying out living room furniture?

Between the coffee table and sofa, you should leave at least 18". That distance provides enough clearance, but still anchors the table to the sofa.

Between the sofa and wall unit, you may find that 20" is just enough to get into the wall unit and still be comfortable while peeking in the door. I recommend actually opening both doors and then see what room you need to maneuver around to access the storage comfortably, especially if you are storing items you will need frequently. Pulling the sofa back also makes viewing the TV easier when the sofa is perpendicular to the TV. If you have an end table or floor light at that end of the sofa, adjust your distance accordingly. Make sure to leave a pathway in and a pathway out of your furniture grouping, it will feel less claustrophobic that way and be more visually pleasing and functional. At *least* 36" if you don't have to worry about access for people in wheelchairs or walkers, etc.

If you arrange your furniture and it still seems too spread out, one thing you can do is find a big area rug to tie all the pieces together visually. This also helps to divide a larger room (like an open kitchen, great room situation.) Make sure the rug touches all furniture pieces of the grouping. But don't spread things out too much, or it wont be comfortable or functional. If the room is very big, consider two separate groupings, the second doesn't need to be more than two wing chairs, a small side table, and a reading lamp (don't forget the rug!) for quiet conversation/reading. Lighting also can make things more intimate.

good luck!