If I want to print out a poster with a 90cm by 60cm dimensions, what resolution do I need and how much DPI? -

Also is there a specific type of paper that I need to use? Thanks!

The trick is finding a printer that will work, and that printer will decide what paper you can use. At 300 DPI = around 760 DPCm (good for everything but photo paper) the width of the image would be 10,600 pixels or so. Using plain white you could get by with just 2,750 pixels, don t go lower. Photo quality should be 600 dpi or better so you are looking at over 20,000 pixels width.