OK, I'm back again. I set the dimensions in the box on making a new document. I set them at 16.4 centimetres and 22.6 centimetres to allow for 3mm bleed all round and 5mm all round to include crop marks outside the image area.
When I chose to view it actual size, the image wouldn't fit on the screen! I checked that I hadn't inadvertently altered the dimensions by pasting in something larger, and the dimensions were still set as I had entered them. But the image is HUGE. I knew it was not just the display which was going crazy because when I came to type in text for the document I was trying to lay out, it needed 95pt text to be readable - and even then it wasn't that big. I don't understand what's going on. When I print that image, even though I have unchecked the box to make it fit to the page, I get an almost A4 image with a very small border - and it is a lot smaller than the "actual size" image on the screen. I'm perfectly happy to believe I'm doing something stupid to cause this to happen, but I'd be grateful if someone can work out what on earth it is I have done....