If you are going to print at 200 PPI, then you multiply the inches in each direction by 200 to determine how many pixels need to be in the image. For example, for an image that will be 30 x 40", this is what you would do:
20 x 200 = 4000 pixels
40 x 200 = 8000 pixels
Then it doesn't matter what resolution setting you choose because it has no effect on the resolution or quality of the image. 200 PPI is probably adequate for an image that size because it will be viewed from a greater distance. But if you insist on printing at 300 PPI then you can do the math to determine how large the image will have to be.