When considering the minimum acceptable resolution to supply photographic images for a graphic reproduction, you need to first consider the intended viewing distance of the image in situ. As a rule of thumb, the minimum viewing distance, which allows the whole image to be viewed in its entirety, can be calculated by multiplying the length of the diagonal of the image by 1.5.
For example, if I am designing a 6m x 3m billboard as in the Kiss Billboard in the photo below, with a little bit of help from Pythagoras, the minimum practical viewing distance would be
1.5 x sqrt(3x3 +6x6) = 1.5 x 6.67 = 10 metres
The following table shows whe maximum resolution in PPI, that a person with 20/20 eyesight, in ordinary viewing conditions can resolve. In other words any resolution higher that the resolution quoted will be indistinguishable at that viewing distance.
|TIFF File Size|
For our 6 metre x 3 metre billboard skin being viewed at 10 metres, you can see from the table above that we are not going to get any better than a file which is 9 PPI (Pixels Per Inch) at the actual output size. So the size of the file we require will be 6x3x0.5 = 9MB, probably smaller that you thought right?
The other thing to point out is that even though the photographic image might be pixilated at shorter viewing distances, generally the text and any vector graphics should be supplied as paths so you will enjoy the highest resolution that the printer has to offer for these vector elements.
It is important to consider the context or situation that the final output is going to be seen. In our billboard example, a 10 metre viewing distance might be appropriate, however what about if you want to do a giant wall mural in an internal environment, rather than considering the context of someone standing back and observing the whole print. You want to consider an observer who is up close. The industry standard resolution for Internet graphics is 72 PPI, from our table above we can see that the viewing distance for this resolution is 1.2 metres, yet you and I know that usually we are sitting closer than half that distance away from our screens. If we to use 72PPI for our 6m x 3m billboard then the Tiff file size would be in the order of 600MB file which is overkill. In reality, the limitation is not going to be the file size as much as limitations of the capturing device. Even your 10 mega pixel digital SLR is only going to give you around 12 MB in RAW format, so unless you are using a number of images, are step and repeating, are adding noise and other filters in an attempt to make a silk purse out of a sows ear, then supplying us with 600MB files is only going to slow everyone down and cost you money in couriering disks. In general keep your file sizes down to under 300MB and then everyone will be happy.