Sizes

Always wondered how big were all these things biologists talk about? :)

1. DNA base pair (bp): ~3.4 Å (340 pm) of length along the strand. Weight: roughly 618 or 643 daltons for DNA and RNA respectively
2. DNA chain: ~2 nm diameter, helical pitch 3.4-3.6 nm, 50 nm persistence length (stiffness) [1]
• See Wikipedia for physical description of the helix. 
3. Human DNA: The haploid human genome (23 chromosomes) is estimated to be about 3.2 billion bases long and to contain 20,000–25,000 distinct protein-coding genes.
1. One genome copy weights about 3pg
2. Each somatic cell has 6.16 pg of DNA (sperm and egg cells have 3.08 pg).
3. The total amount of related DNA base pairs on Earth is estimated at 5.0×1037 and weighs 50 billion tonnes. In comparison, the total mass of the biosphere has been estimated to be as much as 4 TtC (trillion tons of carbon).
4. Protein: hemoglobin: 6.5 nm
5. Virus:
6. Cell:
1. (Red blood cell) 10um diameter of the "disc". Contains ~280M hemoglobin molecules.
2. (White blood cell)
3. Neuron
4. Epithelial cell
5. Endothelial cell
7. Blood vessel:

Schematic representation of the vessels of the cardiovascular system, with the inner diameter, average blood-flow speed, and Reynolds number from Berger et al. [2].
8. Almost every cell, in almost every tissue of a vertebrate, is located within 50–100 μm of a capillary [4]. Check [5] for some introductory but nice reading on capillaries. Can we get a 3D image of the capillary network in a volume?
9. Blood drop: 25 ul
10. Hair:
11. Mosquito: 3-6 mm. 5 mg. Fly at ~1-2 km/h. Wing frequency: 500 bps
• Proboscis (stinger): it is actually six pieces but the one that gets inserted in the skin to suck blood (labrum) is about 30/20 um outer/inner diameter and 1-2 mm long. In the last 50 um goes from 10 um to 1 um diameter.
12. Hypodermic needle: there is a range of gauges:
• 21-gauge (0.8 mm/0.5 mm outer/inner diameters) needles are most commonly used for drawing blood for testing purposes, and 16- or 17-gauge needles (~1.5 mm outer diameter) are most commonly used for blood donation.
• The smallest hypodermic needle typically used (30 gauge) is about 320 μm in outer diameter and 160 μm in inner diameter.
• Thinking to copy something from the mosquito? :) [3] Newer article here.
13. Organ sizes:
1. Brain size: about 1 liter: 10x10x10 cm^3. 10^11, i.e., 100B neurons.
2. 
   

A nice diagram with some of these:

References:

[1] DNA in a material world

[2] Original: Berger SA, Goldsmith W, Lewis ER. 1996. Introduction to Bioengineering. Oxford, UK: Oxford Univ. Press. Extracted from: Microrobots for Minimally Invasive Medicine, Nelson et al.

[3] Fabrication of a micro needle for a trace blood test
[4] Molecular Biology of the Cell, Bruce Alberts et al.

[5] Capillary structure and function in the body