If you've measured the particle size distribution, then 6/D[3,2] is the specific surface area in m2/cm3 (D[3,2] is the Sauter Mean Diameter - see attached) which you can convert to m2/g with knowledge of the density.  This D[3,2] and SSA is actually shown on Malvern Instruments diffraction instruments   There are routes to carry out a dye absorption (or radiotracer) experiment in a manner akin to a BET experiment.  If I recall correctly Gregg and Sing's text may deal with this type of surface area determination - you keep adding dye and measuring the uptake by the lipids tracking the concentration in the dispersed phase (which tells you how much has been absorbed) .  A Langmuir isotherm plot results which when back-extrapolated to zero concentration gives you the surface area.

You can try this...https://volard.wordpress.com/2015/03/22/liposome-calculator/. 

The previous link looks interesting.

Theoretical calculations usually work quite well, but remember that:

a) on one hand, the mean number of bilayers per liposome affects the total surface area of a liposome suspension, and

b) on the other hand, small liposomes have phospholipid asymmetry distribution. Thus, size also modifies the external total area.

Both factors have to be taken into account in a proper theoretical calculation of the total external area of a liposome suspension.