He he he... ok so maybe you already know how to do this. But one of my friend's and I have been super anal about being correct on sizing things correctly since high school.
For anyone that has struggled with getting proportions correct this is a simple little demonstration of the math. It's all about finding a scaling factor. You'll need a ruler and a measuring tape, and if you care about your monitor maybe a printed picture would be good too.
I think it's pretty self explainitory from the picture, but feel free to ask questions if you have them.
For my Serah cosplay I also special printed the fabric for the skirt. I used a drawing package called, inkscape(it's free) to recreate the basic pattern. The pattern at the top and bottom of the skirt are the same so I moved the portion from the top to the bottom to make the building block similar.
I used a site that allows you to print your own patterns onto fabrics. I used happened to use www.fabricondemand.com however there are other places that will do this too. I tried working with vendor to allow them to create the pattern, but they couldn't seam to get the dementions right. I finally insisted on getting the exact dementions the fabric would be for the whole printed peice. I believe it was something like 45" by 72". I knew that my basic pattern needed to be about 5" on one side. I created a blank area in paint shop 4500x7200 in size. I rescaled my basic block to have the long side be 500 pixels, using the keep proportions option. That allowed the other dimension to automatically be the right size. I copied and pasted my block into the blank screen until it was totally covered, making sure each new block lined up with the others. This was easy since Paint shop has a "snap" feature when placeing new objects. Then I sent that picture off to them, they changed it into their format and I approved the file for printing.
Cosplay picture was taken by: Ljinto
I do not own the original Serah picture, it was made by the nice people at squarenix. I have used it for teaching purposes and will in no way profit from it's use.