Concerning the size of everything, here is a "Golden Dawn-esque" example:
"Second, it is traditional to have an altar. This can be a small table covered with black cloth, or the traditional "double cube". A good size for the traditional altar is 36" high by 18" wide by 18" deep. This way you can have an altar that is like two 18" squares on top of one another (as above, so below). It also represents the earth plane because the number of sides of this altar which are external (top, bottom, the four sides of the upper cube and the four sides of the lower) equal 10, the number of the Earth according to the Kabalah.
I made my altar in a very easy way. At an unfinished wood store they sell cubes of press board (also known as chip board) that are 18" x 18" and have one side open. I simply nailed the two cubes together and added a piece of 18" by 36" plywood (1/2" thick) to cover the open sides of the cubes. Upon adding several coats of paint (even with a sealer coat, this type of wood seems to drink up paint), and some wheels to make the altar moveable, and I was finished..."
-Modern Magick: Eleven Lessons in the High Magickal Arts.
When I made my first altar back in 1998, I used the above description. I simply went to my local "DoIt Center", told the man working with the lumber what I needed to make, and we figured out what the measurements of the wood would be once I put it all together (you have to account for the thickness of the wood). So he cut it all up for me, I made my purchase, and went home to assemble my essentially "pre-made" altar. Of course I didn't tell him it was an altar, but I didn't have a saw of my own back then, so it worked out just fine. The finished product had the precise measurements, but no wheels.
That was before I "met" Crowley, who really wants people to use their own measurements, symbols and such, to an extent. When considering the size of the altar, consider what measurements you and/or your wife think are right for you, and design from there. If you have limited space, you obviously will have to design for it.