1. Image sizing refers to the size of the export in pixels. You need to know how large you want to make the print, and then ensure that you have sufficient pixels to support that size.
2. I think it's best to make virtual copies of the image and then crop it to the aspect ratio that you want printed. That way you can be assured that your print will look the way you want.
3. The size of digital images is measured by the number of pixels in the image. And you need to know how to do the math in order to get the right number of pixels for the size print that you want. For example, if you want to make sure a 16 x 20" enlargement is printed at 300 PPI then you need to make sure that the pixel dimensions are 4800 x 6000 pixels. However, many of the printing services are able to interpolate and produce beautiful enlargements with smaller images. Usually when you are ordering the order form will notify you if your image is too small.
4. Don't enlarge is usually selected if you have images of varying sizes (in pixels), and you are exporting to a specific pixel size. Sometimes if you know the you have small images and you don't want them to be interpolated to a larger size you choose this option. If you are talking about size in pixels, you can configure the info display to tell you what the cropped size of your image is.
5.For printing I would set the quality somewhere between 85 and 100. For web and e-mail images I usually drop the quality down to about 70.
6. That's a good rule for calculating purposes. But changing the resolution has no effect on the size or quality of the print or the image for the web. The only resolution that really means anything is the number of pixels. And you need to learn how to calculate what you need. For instance, consider a 4 x 6" print:
4 (inches) X 300 (pixels per inch) = 1200 pixels
6 (inches) X 300 (pixels per inch) = 1800 pixels
Once you understand that concept then the resolution number becomes meaningless.