I've double-decked part of my layout, and the lower level, at left in the first photo below, is at a height of 38.5", dropping down to 36" on the right side of the aisle while the upper level is just under 59".  Most of the upper level is the same depth as the portion below, and in the photo, the depth is 32" along both sides and 38" at the far end of the aisle:

On this aisle, to the right, the lower level is 36" high, and 21" deep, and the upper level 59" high and 25" deep.

I purposely made the upper fascia deep, partly to hide the undermounted lights from an operator running trains on the lower level and partially to hide the upper level's support structure.  In the photos above, the viewing area between levels is 10" along the left side in the first photo, and about 12" for the rest of the double-decked portion.
Almost all of the scenery and trackwork on the lower level was completed before adding the upper level, as it would be difficult to do with the upper level in place.  I will be adding remote control to many of the turnouts on the lower level, but, with some care, they are still manually-operable.

Wayne