  We present a mechanism and show two variants of a method where the average
diameter of spontaneously (barrier-free) assembled single component unilamellar
liposomes is intrinsic, in agreement with Helfrich's theory. It depends only on
the temperature and the lipid type, eliminating kinetic effects or external
forcing normally observed. This provides the first pure system to study the
self-assembly of vesicle forming components, and with a natural length scale it
may have an implication for vesicle size selection under pre-biotic conditions.
