Julianne,
What an interesting question you raise. I'm thinking about it from entirely different viewpoints: The "read" that a given painting possesses, the scale, and the surface quality.
The "read" of a painting, to me, is its ability to be seen well at a distance, and readability is a function of value clarity. Solid massing of values makes or breaks a painting's read. Where values are not clearly differentiated, the viewer needs to stnd progressively closer to see what is going on within the frame. There are paintings that are beautiful close up, but don't hold together as well at a distance.
The other aspect of viewing distance I am thinking of is surface/textural quality...what springs to mind are Robert Johnson's works, especially as viewed in his book
On Becoming a Painter, or on his website.
http://robertjohnsonart.com/.
The title pages throughout the book are luscious, with wonderful color details that are rich and beautifully abstract, and stand on their own as exciting images. From a distance, many paintings may appear to have a very smooth surface, yet once you walk up close to them, they're anything but smooth.
As to optimal viewing distance, the paintings' scale is obviously important, since it is not sensible to view a 6" x 8" painting from 12 feet, nor a 30" x 40" piece from 12 inches. Comfortable to me just means that I can encompass the whole painting visually without having to step back.