Many new homes have “great rooms” that make typical molding and millwork look puny. There are larger variations of common sizes available at the big box home improvement stores (5 1/2″ baseboard to replace 3 1/2″ for example), but sometimes even the largest stock sizes are dwarfed by a room that is kitchen, dining room and living room combined.
I figured out that the way to make large crown molding is to combine several pieces of molding intended for other, smaller applications. 
I bought several pieces of different styles of MDF molding from Home Despot in three foot lengths, and used a chop saw to cut off three inch samples. The samples will stand up on a table, so I can arrange them and get an idea of what they will look like assembled. In the example I am showing here, there are two pieces of baseboard molding on the “wall” and “ceiling” planes, and there are two pieces of crown molding – one for the actual ceiling crown, and the other as a blind for rope lighting.
Once I’ve mixed and matched and arranged a pattern I like with the small cutoffs, I take the remaining pieces (two feet, nine inches) and tack them together so they can be held up against the ceiling in the location I intend to cover, which enables me to get a good judgment of the scale. In this case, it’s simply too big – not only in overall scale, but the descender will interfere with the doorway. So I removed one of the two baseboard pieces from both top and bottom, and ran the spot check again. Ah! Much better!
Later this year, I will be trimming this room, and the research part of the project is now done.