Resolution is the term for the amount of pixels in an inch or ppi. They feel that 200 ppi is sufficient which is what you would set your scanner software to. That's a good starting point.
I don't know what the size in inches of your drawing is,bit if it were 10 x 10 inches, then 10 in x 200 ppi yields a total of 2000 pixels which is what they want. If your drawing is much larger,you can reduce it to 2000 px using the Image Size command once in Photoshop.
So 2000 x 2000 are the on-screen "Pixel dimensions," and ppi is "Resolution" which is how those pixels would relate to a sheet of paper. I'm not sure if your work will be printed or displayed on a web page.
New Document size? Depends what you normally work with. You just manually enter what want,and if you find yourself using the same size,just save it as a custom preset.
The best advice is to scan at a higher resolution that you will ever need. You can always reduce the resolution of the image is too big, but you loose quality if you try to increase resolution.
How big the scanned image will be depends on what you start with. Try a few scans and get one that is at least 2000 pixels in the smallest dimension.