Sorry, but I'm a bit confused now. What is the actual size of your picture: pixel dimensions and resolution? What font size? In your first set of pictures the font size is 12, but in the second set you said 150.
You also said you are going to rescale the picture to A3. Why not just create the picture with that size (or other size you intend to print).
I still think your "problem" is just with the viewing. Have you tried printing it?