I need to print a photo in large scale 3x3m. What size a photo should be, what pixel dimension? What resolution do I need to avoid pixelization? Also what format of file should it be ? PDF?
This photo will stand in a shop window.
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For this application, where the viewing distance is fairly short (a little under 3m, assuming the photo is at the back of a medium depth window), you probably need a minimum of 75 dpi output from a large format inkjet. I wouldn't recommend going below 60 dpi unless the image is low contrast and without much high frequency information like hair or grass.
For a misty scenic shot, you can get away with amazingly low pixel resolution, but not if there is detail. (In my own work for one client, I sometimes have no choice. We are required to use images supplied by an artist's management for poster display outside the theater, and their concept of "high resolution" is sometimes mind-boggling. I've learned by experience what will and won't work.)
At 3m square, then, you would prefer an 81 Mpixel or larger image.
If this will be a fresh digital capture, rent a large or medium format camera with at least an 80 Mpixel back. If you already have a transparency or negative, get an oil drum scan to at least 9000 pixels square; more is better.
If all you have is an existing digital capture, it's hard to suggest a best approach without seeing the image and a more detailed idea of how it must be displayed. It may be that you must use a screened-back image at full size, with a smaller version inset.