Most printshop (2016) print at 300 PPI so so ideally 1800 x 2400 pixels = prints 6" x 8",
Panel |
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1800 / 300 = 6" |
What about DPI? That's for scanning in.
To do fancy collages, you may want to leverage existing templates from websites like Design Algow.
References
What Print Shops Really Want - http://www.rideau-info.com/photos/printshop.html