Hi Irisha, thanks so much! I am glad that all of the information I share is helpful to you. Now about Optivisors, the numbers you mention, 8", 14", and 20", relate to how closely you will need to hold your work to your eyes for the magnification to work properly. I like to recommend 14-inch to my students because this distance allows you to work without having to hold your work too closely, preventing strain on the back, neck and shoulders.
By "magnifying glass" I think you mean what power of lens that goes into the Optivisor. Most people use a 1.75 either with or without their prescription glasses. The greater the magnification, the closer to your eyes you have to hold the work to see it properly.
Just remember that the further away you are able to hold your work and still see the details will prevent pain in your body!
Answer contributed by Dale "Cougar" Armstrong