Consult your printer-documentation. What is the max resolution (dots-per-inch) of your printer?

My fax-printer is able to print with 6000 dpi (low end). That is, if I want to have a picture where each rendered pixel repesents a dot on my printer, and I want to print A4 ( 8.3 inch wide), I need an image with 8.3*6000=49800 pixels horizontally. A3 is exactly times 2 = 99600 pixels horizontally.

But this is theory.

To find the resolution that fits your needs, take a very detailed, nicely colored image (size from above formular). Print it. reduce the pixel-size by 50%. Print again. If you can see no difference: Reduce the size again by 50%. Repeat until you can see a difference. Than add 50% from the difference to the previous print.

After round about 8 prints, you will have the best resolution for your printer.

Sample:
Start with 49800 pixels horizontally.

2. print: 24900, no visual difference to 1
3. print: 12450, still no visual difference to 1
4. print: 6225, DIFFENCE to 3 and pre!
5. print: 6225+(12450-6225)/2 = 9337, STILL A DIFFERENCE to 3
6. print: 9337+(12450-9337)/2 = 10893, NO difference to 3 - maybe the best
7. print: 10893-(10893-9337)/2 = 10115, NO difference to 3 - really the best

Tip: To save ink and time, make 3/4 of the image plain white ....

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