Re: what is the elemental composition of the human body ?

I found several webpages, all of which cited the same list, reprotedly from the now-defunct U.S. Bureau of Chemistry and Soils. The original list shows elemental composition of the human body by weight, not including the trace elements: manganese, zinc, copper, aluminum, cobalt, selenium, chromium, etc. Using hydrogen as a standart, I converted this table into elemental composition by mole to reflect stoichiometry. Then I estimated that the human body is 72% water (an admittedly low estimate) and removed this from the original list, before recalculating the elemental composition by mole, dehydrated. Here are the original table and my conversions:

          Oxygen      65%
          Carbon      18%
          Hydrogen    10%
          Nitrogen     3%
          Calcium      1.5%
          Phosphorous  1.0%
          Potassium    0.35%
          Sulfur       0.25%
          Sodium       0.15%
          Chlorine     0.15%
          Magnesium    0.05%
          Iron         0.0004%
          Iodine       0.00004%
  

          Hydrogen    63%
          Oxygen      26%
          Carbon       9%
          Nitrogen     1.25%
          Calcium      0.25%
          Phosphorus   0.19%
          Potassium    0.06%
          Sulfur       0.06%
          Sodium       0.04%
          Chlorine     0.025%
          Magnesium    0.013%
          Iron         0.00004%
          Iodine       0.000002%
  

          Hydrogen    52%
          Carbon      39%
          Nitrogen     5.6%
          Oxygen       1.6%
          Calcium      0.98%
          Phosphorus   0.77%
          Potassium    0.24%
          Sulfur       0.20%
          Sodium       0.15%
          Chlorine     0.10%
          Magnesium    0.06%
          Iron         0.0002%
          Iodine       0.000008%
  

Now to answer the rest of your questions:

If an average human body would be broken down into its most basic elements and they were then compressed or melted into a single mass how much would it weigh,

Well, the Law of Conservation of Matter says that if you take any object and alter it physically or chemically without adding material to it or removing material from it, the mass remains constant. So a condensed 75 Kg human would have a mass of 75 Kg regardless of what state the matter was in. If the water were removed, one would expect a reduction of 75% of the mass (see above), so a 75 Kg body would dehydrate to around 19 Kg; though water is as much composed of "stardust" as any other components of life.

what would its approximate size be,

The size would depend on the density of the final material, which relates to the last part of the question:

and which common mineral or rock would it most closely resemble?

Considering the amount of hydrogen in both the hydrated and dehydrated forms, minerals are out of the question. If the dehydrated material were oxidized to remove the hydrogen (as water), there wouldn't be enough calcium or other metal cations to form enough of the carbon into mineral carbonates, so most of the carbon would be lost as carbon dioxide gas. So, we go back to the unmodified material. To keep the gaseous elements (hydrogen, nitrogen, and oxygen) from escaping, they must remain bound to the carbon, yielding some form of aminocarbohydrate for the hydrated material or aminohydrocarbon for the dehydrated material. Arguably, by keeping the water, one could conceive of producing a hydrated crystal of aminocarbohydrates: something like [C₆H₁₃NO₅]_n · xH₂O with traces of Ca₃(PO₄ )₂, NaCl, and other "impurities". The closest compound I could find to this was glucosamine, which can form crystal salts with sulfates and phosphates, the hydrated forms of which would have below average specific gravities (probably around 2.0 g g^-1), even with the impurities. This would give a displacement for the crystalline human around 38 liters - about half the size of the original living body.

It's amazing how many factoids one can generate from a 70 year old list!