You might want to look at a webquest used by another teacher on the pyramids of Egypt---just for ideas of how to organize:
http://users.wcvt.com/tiggr/ Mr. Pitonyak's Pyramid Puzzle. He divides the class into groups of 4, each with an engineer, a project director, a secretary, and an accountant. He also gives them a specific question to answer and talks about grading the project.
There are actual measurements given here: http://en.wikipedia.org/wiki/Origins_and_architecture_of_the_Taj_Ma... on a Wiki site.
You may or may not like this lesson plan The Mathematics of the Taj Mahal http://peer.tamu.edu/LessonPlan.asp?id=95&file=activity but at leat you can download the MiniModule [see top of page] and then choose to look at the Worksheet; you'll find some measurements there.
A small thought: If the measurements are given in metric, you might want to use that for your basic scale, instead of converting to inches, feet, etc.
If you have access to a computer projector, computer lab, or some laptops, I would suggest building it virtually using Google SketchUp
Its a great (and free) program that I use with my students. They can either attempt to build it from scratch (with fairly acurate online building tools) or can look at some pretty cool 3D models that are already posted.
If you decide to go this route and have any specific questions, please let me know.
What an interesting project! My advice for the building portion of this project would be to use safe materials and tools. Especially with middle school students, it is amazing how quickly seemingly innocuous materials can become safety hazards!