Two Cosine Functions: Altering dimensions/scaling

a[cos(bx + c)] + d

There are two cosine functions. One is higher than the other. There is a fixed domain such as 10 < x < 10 so that the area between the curves creates an image.

How do I, for example, double the dimensions of this image? And how can I play around with the variables of each curve to create any image size that I want? Thx