"Set the original at quarter size. Any images excluding Vector files should be no less than 300 dpi (resolution.) Once outputted at bureau, final res would be around 72 dpi which is roughly standard for large format graphics."
That's all Greek to me, as they say, but he knows his stuff, so I hope it makes sense to you!!
Thanks, that is quite useful, but do you (or anyone else know) whether how many pixels are in the image matters too? (i.e. if it was 3000 x 2000 pixels big etc)