  We present a comprehensive high spatial-resolution imaging study of globular
clusters (GCs) in NGC1399, the central giant elliptical cD galaxy in the Fornax
galaxy cluster, conducted with HST/ACS. Using a novel technique to construct
drizzled PSF libraries for HST/ACS data, we accurately determine the fidelity
of GC structural parameter measurements from detailed artificial star cluster
experiments. The measurement of rh for the major fraction of the NGC1399 GC
system reveals a trend of increasing rh versus galactocentric distance, Rgal,
out to about 10 kpc and a flat relation beyond. This trend is very similar for
blue and red GCs which are found to have a mean size ratio of
rh(red)/rh(blue)=0.82+/-0.11 at all galactocentric radii from the core regions
of the galaxy out to ~40 kpc. This suggests that the size difference between
blue and red GCs is due to internal mechanisms related to the evolution of
their constituent stellar populations. Modeling the mass density profile of
NGC1399 shows that additional external dynamical mechanisms are required to
limit the GC size in the galaxy halo regions to rh~2 pc. We suggest that this
may be realized by an exotic GC orbit distribution function, an extended dark
matter halo, and/or tidal stress induced by the increased stochasticity in the
dwarf halo substructure at larger Rgal. We match our GC rh measurements with
radial velocity data from the literature and find that compact GCs show a
significantly smaller line-of-sight velocity dispersion, <sigma(cmp)>=225+/-25
km/s, than their extended counterparts, <sigma(ext)>=317+/-21 km/s. Considering
the weaker statistical correlation in the GC rh-color and the GC rh-Rgal
relations, the more significant GC size-dynamics relation appears to be
astrophysically more relevant and hints at the dominant influence of the GC
orbit distribution function on the evolution of GC structural parameters.
