Most printshop (2016) print at 300 PPI so ideally 1800 x 2400 pixels prints 6" x 8",
1800 / 300 = 6"
2400 / 300 = 8"
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