Practical limits on the size of orbiting objects: could two pebbles orbit each other
As Brandon mentioned, two small objects couldn't orbit each other near a significant gravitational field. The Hill Sphere "approximates the gravitational sphere of influence of a smaller body in the face of perturbations from a more massive body." Therefore, your pebble's Hill Sphere would be too small to permit orbits near Earth. The Wiki article has a calculation showing that an astronaut couldn't orbit the 104 tonne space shuttle 300 km above the Earth since the shuttle's Hill Sphere was only 120 cm.
If there were no external influences like the gravity from the Earth and stars and no light radiation to push things, then even tiny objects could orbit each other.
Assuming somewhat constant densities (which is generally true for small objects), the mass of an object grows as the cube of the radius: $Mass \propto r_{}^3$. The gravitational strength falls as the square of the distance which must be greater than the radius: $Force \propto \frac{1}{r^2}$. Following this reasoning you can expect the attraction and therefore the orbital speed to roughly increase proportionally as you increase the size of the objects. Small objects means a low attractive force and therefor a very slow orbital speed.
This will break down for microscopic objects though. At those sizes the uncertainty principle would start to have an effect (not to mention other forces like Coulomb repulsion).
I haven't done the calculation but I suspect if small objects on the order of a few inches in diameter would need to be outside of our galaxy for the gravity between them to dominate over the gravity of the galaxy.
Anything with mass will orbit around anything else with mass. Gravity is of infinite range - a proton on your nose knows about a proton in the Andromeda Galaxy.
What's interesting is to see if the orbits produced mean anything sensible. This spreadsheet extract shows some orbital periods (T) for some common and hypopthetical pairs of objects. The point to note is that the orbital period depends strongly on the separation - the closer they are, the faster the orbit. The last row is the two rocks from the question. If they were 1m apart, they would take two days to orbit each other at about 0.4 mm/s.
G=6.67E-11
m1 (kg) m2 (kg) r (m) mu F (N) v (m/s) T (secs) T(hrs) T(days)
7.00E+22 6.00E+24 3.84E+08 4.05E+14 1.90E+20 1.02E+03 2.35E+06 650 27.2 earth-moon
6.00E+24 2.00E+30 1.50E+11 1.33E+20 3.56E+22 2.98E+04 3.16E+07 8780 365 sun earth
1.50E+21 1.30E+22 2.00E+07 9.67E+11 3.25E+18 2.08E+02 5.71E+05 158 6.61 pluto charon
1.00E+09 1.00E+09 1.00E+03 1.33E-01 6.67E+01 8.17E-03 5.44E+05 151 6.30 million tons 1km apart
1000 1000 100 1.33E-07 6.67E-09 2.58E-05 1.72E+07 4780 199 1 ton 100m apart
10 10 100 1.33E-09 6.67E-13 2.58E-06 1.72E+08 47800 1990 10kg 100m apart
10 10 10 1.33E-09 6.67E-11 8.17E-06 5.44E+06 1510 63 10kg 10m apart
10 10 1 1.33E-09 6.67E-09 2.58E-05 1.72E+05 47.8 1.99 10kg 1m apart
0.02 20 1 1.34E-09 2.67E-11 3.65E-05 1.72E+05 47.8 1.99 1"-10" rocks 1m apart