If you're curious about how big is the asteroid we'll target (from Wikipedi): it's a moonlet orbiting another asteroid; "It measures approximately 0.17±0.03 kilometers in diameter compared to 0.75±0.1 kilometers for its primary"
No mention about mass though, maybe because we don't know what the asteroid is made of?
Knowing the asteroid's composition isn't required to calculate the mass. This can be derived by the orbit. So, the total mass of the binary system (main and moonlet) is known, but the mass contribution of each isn't. Assuming though that they have the same bulk density, M2/M1 = (D2/D1)^3 ~ 0.0116 and considering Σ Mi = M, then M2 ~ 0.0115M.
No mention about mass though, maybe because we don't know what the asteroid is made of?