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What is "image size" in CG?
In computer graphics, image size is always referred to as the amount of pixels the image is made up of, whereby the pixel is the smallest individual color element of the image.
When rendering, the computer is calculating what each individual pixel should look like. More pixels means better detail, but also more calculations and ultimately longer rendering times.
This is why a "4K" image with a size of 3840x2180 pixels is more expensive than an "HD" variant (1920x1080 pixels).
What is "image resolution" in CG?
Technically, image resolution denotes the on-screen pixel density of an image. It is expressed in Pixels Per Inch or ppi - the amount of pixels per square inch. Confusingly, the term "resolution" often gets used to indicate image size.
What does "dpi" mean?
300dpi refers to the world of printing, where dpi (Dots Per Inch) indicates the amount of ink dots a printer lays down on 1x1 inch of paper. Therefore dpi is only truly relevant if the image is to be printed. However, as the terms are complicated to explain, in CG the conversion is generally taken as being 1 to 1: 300dpi equals 300ppi.
As said, it is the image size that determines image quality. However, many customers and designers will refuse to accept a resolution below 300ppi, as traditionally 300dpi is considered the correct resolution for printing an image at high quality.