Good News: this kind of cape uses a lot less material that what you are used to, a yard & half (maybe two if you are tall), of each outer fabric & lining.
Bad News: It takes a bit more patterning than the classic circle capes.
Good News: The patterning is pretty simple. You can use a shirt you own to build the pattern from as you only need shoulder shaping and the collar, which because it is a full wrap around the neck is actually just a straight piece. You could invest in a shirt pattern that has a Mandarin collar and alter it if the idea of building a collar pattern scares you.
For the length and shaping at the bottom, I'd do a scale model to figure it out. In sculpture we use the fancy term maquette
. Find a doll (Barbie, American Girl, anything with somewhat close to normal proportions. It doesn't matter what size the doll is. Things with big heads like Blythe dolls won't work).
Grab a scrap of fabric or light paper (tissue, newspaper) and cut a rectangle. Tape one end to the top of her shoulders, with the edge right top center of the shoulder (where a shirt seam would land).
Use a marker and start drawing on the scrap until you get something that feels close to correct. Don't worry if the points are different widths, just focus on length in relation to the doll & width of the cape in different areas (shoulders, waist, knees).
Take it off, trim it up, put it back on.
Mark it again and trim as needed. You might want to trace version 1 onto other paper/ fabric to modify for version 2.
Do this until you feel you have it, for me that is often three progressively better tries.
Go figure out or copy off the shoulder & collar portion of your pattern. Make a paper copy if you are using a commercial pattern. You can trace pattern tissue by using a Sharpie and drawing straight over the printed tissue lines. It will bleed through to your other paper, making a super easy to work with extra pattern piece.
Now comes the math part O.o
Measure your height from shoulder to floor (in inches or centimeters, don't use feet or other bigger increments). Measure the dolls height from shoulder to floor. Measure the cape maquette from shoulder to bottom edge. solve for X.
I cannot for the life of me get the algebra to type correctly in this post, will add a visual.
You can do a second set of algebra for the width OR draw a graph on the maquette based on the initial measurements and simply use a grid to transfer. http://appletiniphotography.com/diy-...rge-a-drawing/