I took an arial photo with a Gopro hero3 12mp, and I'm going to to print it at
90cm x 60cm. I'd like to know how it will look like before I print it.
I know about the
print size option in Photoshop but that doesn't make sense to me.
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First you need to prepare your file to the necessary size and evaluate the effective dpi.
Then you can take one fragment (A4 or Letterhead size) and print that on the equipment you will be using as a test.
I don't see any other way to understand how it will look when printed.