I've got an image 4,200 x 4,200 pixels. I want to have it printed off as the biggest poster size I can get. How do I calculate what the largest size would be before I get pixelation or fuzziness. It's a very detailed image so it needs to retain the quality. Below is a link if that provides any details I've excluded.
Some initial observations:
You do not read a magazine at 2 m. from you. Lets think that you read it at 30 cm.
But you can see a poster at 60cm, or a bus stop add at 1 or 2 m.
As you get further away you need less resoluiton on your print.
So the first point is define your viewing distance.
If what you want is a wall mounted image on your room, I say it is arround 100-150 ppi. For me 100ppi is enough, so lets do some math:
4200px/100ppi = 42 inches.
4200px/150ppi = 28 inches.
I would recomend that you print a small fragment of the image on a letter sized paper and then you decide the final resolution.