Frank's iconic cape is black satin lined with silver lamé. It has shoulder pads and there is pleating across the back under the collar to accommodate its fullness. My best guess is that hem is at least 12 to 15 feet around.
He keeps it closed across his chest with his hands, but if you're going to be doing a lot of walking around before your reveal you might want to consider a small closure at the neck. 
The collar can be stiffened with buckram (available at most fabric stores, typically used for hat forms) and, if needed, a few well-placed pieces of nylon boning. The collar looks like it might have 5 obvious boning channels spaced evenly around.
When choosing your fabrics, consider two fabrics of a comparable weight. Otherwise a heavier fabric is likely to hang unevenly over its counterpart as it sags more under its own weight.
Simplicity Pattern #2499 has a nice 5-piece cape (6 with the collar) where the center back piece could be widened more to accommodate some pleating. I know on large-scale projects like this it's tempting to cut some corners, but make sure to cut your fabric on the grain as the pattern specifies or your cape will drape oddly, as well as pucker at the seams.
