How would I go about creating map entities (by programming) based on dimensions entered by a user?

I'm only interested in creating floors and walls with this method - so only rectangular objects will be used.

If there's info already in the forum, I couldn't find it using a search.

Example:
A user wants to create a floor that is 20 * 30 * 1, or a wall that is 20 * 30 * 8.
The user enters the Width/Length/Height into text boxes.
The "code" creates the object (using a single texture) and places it on the map.
(I can determine the placement of the object)

I am open to creating a WMP or WMB, which ever would be easiest.

(Yes, I've looked at the file format docs - just a little problem with where to get started...)


Thanks in Advance,

~Neut.