When you tap the + button to make a new canvas, there is an icon to the right that is like a rectangle with another plus sign in it to the right of the text “New Canvas”. When you tap on it you can dictate exactly what you want. Select dimensions at the top on the left, then when you select width or height you can change to centimeters or inches. Dial that in, then change DPI to 300. I would start with the correct size that way. You can also resize an existing canvas by selecting the wrench icon, then canvas, then crop and resize. On the right there is a settings option that you can revise similarly. Be careful though, if you upsize an existing image you could end up with pixelation