There are two valid answers to this, depending on whether we look at this as a relative issue or a subjective one. As a relative issue, we use math to compare number of file pixels versus output resolution. Subjectively we look at quality as simply a matter of personal taste – what I like to call “The quality to pain threshold”. Or how big can we go before the quality drops to where it becomes painful to look at or pay for.
First, in either point of view, image quality is more than just the number of pixels contained in the file. For a simple example; a modern 24 mega-pixel file shot out-of-focus will be of lesser quality than a properly focused 4 mega-pixel file.
Let’s look at the relative approach first, since most folks like easy and firm answers, such as 2+2 always = 4, and George Washington was the first US prez.
The easy answer is achieved with simple math:
File pixel dimension ÷ minimum input resolution = output dimension.
2000÷300 = 6.66″
3000÷300 = 10.00″
The largest maximum quality print size would be: 6.67″ x 10.0″
If your printer recommends a minimum of 150 ppi:
2000÷150 = 13.33″
3000÷150 = 20.00″
The largest minimum quality print size is 13.33″ x 20″
If your file is from a 24 mega-pixel camera with dimensions of 4000×6000:
4000÷150 = 26.66″
6000÷150 = 40.00″
The largest minimum quality print size would be 26.66″ x 40.00″
With the subjective approach, there are limited fixed answers. The size of output is usually limited by one or more of the following factors:
* The physical limitations of the printing device.
* Your budget.
* How ugly you are willing to accept it.
At some point the cost of the print will break your budget. That is a hard and fast limitation. So that’s easy – you can print as big as you want to go as long as you can afford the print.
The printing device or medium will support a maximum specific size. For instance, some ink jets will not print any larger than 40″ wide, but they will go several hundred inches long. You can’t go any larger unless you pick a different printing device or you print in multiple tiles and deal with matching the seams. If you are willing to do the latter, then your budget is again your limit.
The subjectivity comes in with your opinion. How big is too big before the quality drops below your level of acceptance – your threshold of pain. Or you might call it the “Yuck factor”. When you get to a level of enlargement that degrades the quality to a point where you don’t like the results, you have hit your threshold of pain. In essence, you see the print and say “Yuck! That’s one ugly print and I’m not willing to pay money for it”
What does the yuck point look like? I can’t answer that for you, only you can. My level of acceptability may be different than yours. A professional’s need for quality is likely higher than that of the average consumer due to experience and training. Because of this experience, the professional will usually hit his/her level of pain sooner than the consumer.