Do ghosts have brains? I believe so!
To scale to the size you need you could print the first layers of the model and test it.
If you can't do a size test, you may print it in the original size. I have tried the ghost costume on some of my friends and its children (7 and 10 years old) and it has worked. If unfortunately the size is too big for a kid, you could glue foami strips inside to get a perfect fit.
It has two holes to add straps. On the ghost costume the straps are placed under the sheet.