Calculating the pixels of your image
So let's calculate your photos, as we understand not everyone has an iPhone 6s, right? If you want to calculate your smartphone's maximum high-resolution print size, divide your image's horizontal and vertical pixels by 300.
So, for example, an image taken on the iPhone 6s has a 12-megapixel camera. That means any photo taken with it will be 4032px by 3024px. (See Our Smartphone Guide). Take these numbers and then divide them by 300, and you get 13.44 inches (34.13cm) wide by 10.08 inches (25.60cm), which is almost an A4 size which is a very decent larger size!
Note: The calculations are based on inches, as this is the standard measurement conversion for pixels (px) and guides. If you want to convert this into the metric measurement, multiply it by 2.54 to give you the cm size.
For example 13.44 inches x 2.54 = 34.13cm