Surface Chemistry - Dipole Moment of Surfactant with Water and Phases of a Monomolecular Layer
Wow, that is a question! 


Surfactants (aka tensioactive compound) have a dipolar moment at the air-water interface but that is not enough to characterize their behavior. There is a cohesive force between their lipophilic tails (due to Van der Walls force) and there is also a cohesive force between their polar head but only because water of metal ions can make a bridge between identically charged moieties.
I will address only your main question which seems to be "how to know the size of molecules in a monolayer"? Well, a surfactant floats at the surface of water. So you can make an apparatus filled with water and with a movable barrier which will move across the surface to slowly reduce the overall surface available for a tensioactive compound. We also know how to measure variations in the "tensioactive power" (whatever its name is) which depends on the number of molecules at the surface.


Using such a device, we can create a monolayer of a lipophilic compound, reduce the whole area (which we call a "compression") and detect the resulting effect (which correlates to the number of molecules at the surface). Whenever the available surface becomes too small for all the molecules to be nicely arranged at the surface (i.e. the "collapse").