Help for a novice Fresco user to print at a specific size

Advice appreciated.  I am a novice and new to Fresco.  I need to print artwork at a specific size (15cm x 20cm on this occasion) and although I can see where to create the canvas at a certain size, I cannot seem to get it to print out at the desired size.  I am working on an iPad and cannot seem to change the printer settings when I select print either as there is only an A4 paper size and no option available for changing any size related settings.