300 PPI is the de facto industry standard for printing resolution.

When we talk about how big you can print your scans, we simply divide the resolution by 300 to get the number of inches. Hence a 5400 by 3600 pixel scan can be printed to 5400/300 by 3600/300 or 18 by 12 inches. Easy.

If your original image is sharp, then realistically you can drop below 300 PPI and get excellent results. 240 is probably a more realistic figure.

Epson printers realistically produce their best prints with 240PPI or higher, Pegasus machines require 250 PPI, and Lambdas need 200 PPI. So in fact you can generally print significantly larger than we state, without any real visible loss of quality.