What canvas size?

[Class1]

Hi, Excuse my ignorance but can anyone explain how I work out what size pixels i need to enter in relation to my canvas size? For example I want to create a letter head on A4 paper, what pixel size would i put into the canvas size option. Is there a simple formula so i can set it to whatever canvas size I require?
Thanks in anticipation. :confused:

[KD-did]

Setting canvas size can be set either at the start of a project or at time of exporting . Either drag canvas corner to the desired size or under Modify click set canvas dimentions and add numerical size .

[Class1]

I know how to adjust the canvas size, what I don't know is what values to enter for an A4 canvas. I am wanting to create a letter head the full width of A4. What is the pixel size for A4.
Thanks

[Shelly]

595x842 on 72 DPI and 2479x3508 on 300 DPI.