Jules,
for large format printing operators these Photoshop
posters are a horror.
If it is a poster with text, line art and separate
photos then it should be made by InDesign.
The photos should have about 200ppi resolution. Mostly
enough for the rasterizer (RIP) and good for near view.
If the poster has additionally a background raster image, then it should be made as well by InDesign (!).
Such a background ist often little saturated and without detail. 75-100 ppi would be sufficient, as Bob says.
Not so for the accurate images, especially not for product photos.
Now we see: Photoshop is the wrong tool. Here we can
define only ONE resolution for all raster images.
Use color space sRGB for raster images and define text+
line art by CMYK, using any offset space for coated paper, like ISO Coated.
The inkjet colors will be near to your design (or even
accurate, if the printing operator honours your definitions).
In your design you should think in pixels per inch for the final size. Just provide images with enough pixels (simple mental arithmetic).
An InDesign file can be made for the final size and then Exported as PDF with these settings:
Downsample to 200ppi if above 200ppi.
No compression or lossless compression ZIP/LZW (not JPEG). Embed all fonts.
Leave colors unchanged.
The original size PDF can be test-printed for instance
by an A3 toner printer (297mm x 420mm) by scaling or
automatical page fitting. It’s NOT necessary to downsample anything for this test.
Best regards –Gernot Hoffmann
who is occasionally a large format printing operator:
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http://www.fho-emden.de/~hoffmann/hagiasophia.html>