Hi Steve,
Going "bigger" may make it easier for you to paint, but it also means you need to consider many, many small details on the sculpt that would not otherwise be visible in a smaller scale (for example, the red veins in the eyeballs are visible in 1/5th scale but not in 1/16th).
As Chris points out, the Naked Army range is 315mm or 1/5th scale, which is pretty big compared to 120mm (1/16th scale), and is superbly sculpted, but they don't do busts (not yet anyway). I would recommend you check out Alan Mackie's work with Naked Army figures if you want to see what can be achieved in large scale- he's the master not me.
As for going to a bigger scale, I wouldn't recommend it unless you're prepared to paint in all the extra details that are impossible to see in smaller scales. In 1/5th scale, you need to remember that every minor detail is visible to the naked eye, and you can't get away with some of the short cuts that we often employ when painting in smaller scales.
You may find that your failing eyesight is greatly improved by using magnifier glasses and stronger light sources when you paint.