What are the most cost-effective print sizes

A6 product – 148mm(height) x 105mm(width)

A5 product – 210mm(height) x 148mm(width)

A4 product – 297mm(height) x 210mm(width)

A3 product – 420mm(height) x 297mm(width)

You can go any size you would like but print sheets come in set sizes and the printer will bill you for excess paper that they trim away