Phospholipids in membranes usually have a fatty acid chain length of between 6 and 14 I believe (more sure about the higher end than the lower). Lower than 6 and the chain isn't long enough to interact fully with the other nonpolar chains in the hydrophobic inner portion of the membrane whereas longer than 14 can be too long and invade the other side. As for the actual measurements of how large, a membrane (without proteins) is about 20 angstroms (1 angstrom = 1x10^ -10 meters) wide, and consists of a bilayer, so I'd guess around 10 angstroms average for one phospholipid.
