Here is a link to a site I've mentioned multiple times on the show that appeals to the SciFi fluff nut in me.
It, better than any other site I've ever seen, demonstrates visually exactly how large warships in 40K fiction truly are. What inspired me to write this post was the book cover from A Thousand Sons by Graham McNeil and Cain's Last Stand by Sandy Mitchell (Alex Stewart).
Those are battleships and cruisers in orbit. They're big, really friggin big. Big enough to make out individual details of the ships from the planet's surface. But is that an accurate portrayal or just another example of artistic exaggeration? After all isn't orbit really far away? The largest man made object currently in orbit is the International Space Station and every time I've seen it overhead it was nothing more than a bright fast moving dot. See for yourself on a clear night.
So I began to try to figure out just how big do 40K starships appear during an assault/bombardment when they enter low orbit? To answer this out I needed to find out how high assault/orbital bombardment orbit was. My answer was found in chapter 19 of Cain's Last Stand when he describes a Chaos cruiser needing "....about an hour. More or less." to line up an orbital bombardment shot. That duration of orbit around what's described as an Earth sized planet would put an assault/bombardment orbit at just under 150 miles high. If a Retribution class battleship is 7.5 KM long then it's distance during an assault/bombardment would be about 20 times it's length from the viewer.
Do you want to see with your own eyes just how big these ships are? Open this image in another window and leave it full sized, walk back about 14 feet and turn around. This is how big it would appear to the sorry sap on the receiving end when it's firing shells and assault craft are being launched,
Does that put the size of these things into proper perspective? Black Library book covers may be a bit on the dramatic side but they're accurate with the background imagery.