Size of humankind
How large is humankind in volume? Surely we're many, so our volume must be huge, isn't it?
Volume and mass of all living human bodies
It seems from Wolfram Alpha that the average human volume is around 66.4 liters. The current human population is 6.93 billion people.
Therefore all humankind is 4.602 * 10^11 liters of stuff, which correspond to a cube with a side of 772 meters or a sphere with slightly less than 1 km diameter.
This roughly corresponds to a couple of hills or a small lake. There is plenty of icebergs larger than that. We would be barely a blip on a map of the planet: Lake Garda in Italy is more than 100 times larger. Wolfram Alpha tells us that it's 0.92 times the water in the Sydney Harbour, but it hardly seems suggestive to me, perhaps because I'm not Australian.
The mass is unimpressive as well. Here Wolfram Alpha is a bit contradictory, but 62 kg seems a reasonable value given the volume above. This means that we talk of 4.3*10^11 kg (funnily WA tell us it's 1.4 the mass of all humans alive, but it gives no clue on how it arrived to this value). But WA tell us something cool in comparison: there is 23 times more gold in the oceans than human mass.
Given both values, if we had a sphere the same mass and density of human beings in space, it would be a miserable body like a small asteroid, with a lousy gravitational field: 0.115 mm/second^2 of acceleration. The escape velocity would be of little more than 1 km/h.
Volume and mass of human components
Ok, now let's imagine to spill blood from every living human -a kind of Vampirefest. How big a red lake would we fill?
Answer is, a really small one. The average volume of blood in a human is about 5 liters; this means that we have a cube of 326 meters side of human blood on Earth. Yes, it is 13860 swimming pools, but it is a not-so-impressive lake the same. Lake Trasimeno, in Italy, is estimated to be 586.000.000 cubic meters. It is therefore about 17 times larger than the humankind blood volume. A blood lake with the same average depth of Trasimeno would be about 3 km wide in diameter. Would make for nice pictures, probably.
Corresponding mass (for a blood density of 1.06 g/cubic cm) is 36.72 millions of tons, and Wolfram Alpha isn't afraid to tell us this is:
- 0.17 times the yearly trash production of USA
- 6.1 times the mass of the pyramid of Giza
According to Wikipedia, every human produces from 0.75 to 1.1 grams of tears every day, slowing with age. Taking the lower limit (0.75) this results in 5.2 million liters of tears every day - a bit more than 2 Olympic swimming pools. So, when you see a swimming pool, now you can imagine it as half the tears of the whole humankind.
Related to the issue of blood is that of iron content. Our WA tells us, again, that it is 0.06% in mass. Given the mass above it turns out we're talking of a pretty interesting quantity of iron: 25800 tons (which is about the mass of Titanic). So let's sink a ship in our hypothetical Blood Lake.
How much DNA is in a human body? It seems that on average we can talk of about 500 to 700 g - from this thread which is hardly a reliable source, but seems reasonable. This means we have about 3.5 millions of tons of human DNA walking on Earth.
Comparison with other biological entities
The overall mass of bacteria on Earth is estimated at roughly 5*1012 tons. Assuming water density we have a cube with a side of 17 km. This is probably slightly overestimated (bacteria should be denser than water), but even a cube of 12 km side would make the volume of humans look ridicolous (but it would still be insignificant with respect to oceans; it would be the volume of a big lake - WA says it's less than half Lake Superior ).