In terms of size? I ask about the “average” because clearly it would depend on the specific hormone in question.
But approximately how large is the average hormone?
Size as in number of atoms or size as in dimensions?
Not sure how you can determine what an “average” hormone is.
A few are modifications of an amino acid such as epinephrine or thyroxine.
Several others are modifications of a steroid ring configuration. Testosterone, cortisol, and estradiol are in this category.
And a few such as prostaglandins are eicosanoids, a modified fatty acid.
Some are **peptides **such as insulin or oxytocin.
If I remember my biochemistry, very broadly, those four groups (amino acids, steroids, eicosanoids, and peptides) are listed from smaller to larger size. But one peptide may be MUCH smaller than another. And there may be some overlap between groups.
In terms of nanometers/micrometers of their diameter/radius/weight?
But if you took all the hormones which the human body uses: steroids, peptides, thyroxine derivates, you name it…, and added them up to look at their individual sizes, what would the actual size of your namely AVERAGE hormone be?
I know it’s a whacko question, but I’m having trouble learning about hormones without being able to imagine the incredibly minute size I assume they must have.
So? 
Starting from Klisterhjernen’s info and given that I am NOT a chemist, nor do I play one on TV …
Insulin is a peptide. It’s formula is C[sub]257[/sub]H[sub]383[/sub]N[sub]65[/sub]O[sub]77[/sub]S[sub]6[/sub] with molecular weight: 5807.57 (from http://www.sigmaaldrich.com/catalog/search?interface=All&term=insulin&lang=en®ion=US&focus=product&N=0+220003048+219853269+219853286)
Epinephrine is an amino acid has formula C[sub]9[/sub]H[sub]13[/sub]NO[sub]3[/sub] with molar mass 183.2044. (from Epinephrine Properties, Molecular Formula, Applications - WorldOfChemicals)
So for real round numbers insulin is on the order of 30 times heavier than epinephrine. Physically insulin is shaped like a tangled yarn ball, whereas epinephrine is more like a sperm, a blob with a curly tail. A straight mass/volume calc would give the “diameter” of an insulin very, very roughly 3x the diameter of an epinephrine. Given the amount of empty space entailed in folding I’m gonna WAG it at more like 4x.
So now we know (very very roughly) how big they are relatively. How big are they absolutely?
A carbon atom is about 140 picometers (pm) across and they bond together at roughly 140 pm spacing as well. (WebElements Periodic Table » Carbon » radii of atoms and ions)
With a lot of arm-waving, unstated simplifying assumptions, and very little science that says an epinephrine will be something around 1000 pm across. That’s one nanometer. The wavelength of visible light is roughly 700 nanometers. A red blood cell is roughly 7000 nanometers in diameter. Human hair ranges from roughly 20,000 to 200,000 nanometers in diameter.
So an epinephrine is 1/7000th the length of a red blood cell (RBC). An insulin is maybe 1/2000th the diameter of an RBC. An insulin is somewhere between 1/12,000 and 1/120,000 the diameter of hair.
Bottom line: even huge molecules are tiny compared to cellular structures. Which are themselves tiny compared to macroscopic parts of you. Which are tiny compared to you.
Disclaimer: I am not a chemist and there are several crude assumptions in the above that may make the results off by a couple (?!) orders of magnitude. I await a real chemist to laugh at my high-school level back of enveloping and give us the real info.
Hmm, my go-to reference for protein structure doesn’t outright state the size of insulin. It does state the size of the unit cell when insulin crystallizes (see bottom right, under "experimental details), which should be close enough. Going by that, the approximate size of a single insulin molecule is 8.2 nm x 8.2 nm x 3.4 nm. Not far off LSLGuy’s estimate of a 3-4 nm diameter.
This poster (34 x 22", 23MB PDF) shows various biological molecules at 3’500’000-fold magnification. Drawn at the same magnification, a bacterium would be the size of a bus, a mammalian cell the size of a large building and a man as tall as the radius of the earth. The longest cells in humans, motoneurons reaching from the spinal cord to the muscles of the lower leg, would at this magnification reach from Zuerich to Eriwan (Capital of Armenia). On the poster you can see depictions of the peptide hormones glucagon, insulin and epidermal growth factor, which are some of the smallest proteins depicted. Non-peptidic hormones such as steroids, thyroid hormones or prostaglandins and thromboxane are roughly the size of Adenosin triphosphate depicted in the top right corner of the poster.
Whoops, looks like I didn’t understand what “unit cell” meant, it must include several individual insulin molecules. A quick-and-dirty measurement of the insulin structure shows that it’s about 2 x 3 x 4 nm.
The asymmetric unit of pdb entry 4INS is a dimer, the longest distance across the dimer is about 4 nm, the individual insulin molecule is half of that. The unit cell contains 9 dimers, that is, 18 insulin molecules. In the crystal, the insulin molecules form hexamers held together by two Zn2+ ions.
While we’re issuing corrections, I mistakenly credited the OP Klisterhjernen with the partial answer as to hormone categories. That was actually Iggy’s helpful post. Sorry Iggy.