Courtesy of Rob Hafernik:

Here's the description that Rex Stout wrote of the brownstone and office:

The old brownstone on West 35th street is a double-width house. Entering at the front door, which is seven steps up from the sidewalk, you are facing the length of a wide carpeted hall. At the right is an enourmous coat rack, eight feet wide, then the stairs, and beyond the stairs the door to the dining room. There were originally two rooms on that side of the hall, but Wolfe had the partition removed and turned it into a dining room forty feet long, with a table large enough for six (but extensible) square in the middle. It (and all other rooms) are carpeted; Wolfe hates bare floors. At the far end of the big hall is the kitchen. At the left of the big hall are two doors; the first one is to what Archie calls the front room, and the second is to the office. The front room is used chiefly as an anteroom; Nero and Archie do no living there. It is rather small, and the furniture is a random mixture without any special character.

The office is large and nearly square. In the far corner to the left (as you enter from the hall) a small rectangle has been walled off to make a place for a john and a washbowl -- to save steps for Wolfe. The door leading to it faces you, and around the corner, along its other wall, is a wide and well-cushioned couch.

From this I gather that the office is roughly thirty feet square (since it plus the front room, which is "small," must balance the forty feet of the dining room). My guess is that old Rex wasn't all that sharp on distances. Forty feet by twenty is a hell of a large dining room (I know people who live in two bedroom apartments that have fewer than 800 square feet, which is forty by twenty). We know it's twenty feet wide, because it was originally a double house and the dining room plus hallway much balance out the width of the office. A table for six would be lost in such a space - you could probably put a table for thiry in a dining room that size!

I still can't quite visualize the stairs and elevator. Do the stairs circle the elevator? Seems unlikely since the elevator was put in later (after Archie was hired, since he knows it cost $7000 to install). Do the stairs go up towards the rear of the house, over the hallway (seems most likely) or do they go up towards the side (over the dining room), meet a landing and reverse?

Sketch:

Another sketch of the offic, courtesy of Kim Blyth:

Another sketch of the offic, courtesy of Wally: