  Be stars are rapid rotators surrounded by a gaseous disk envelope whose
origin is still under debate. This envelope is responsible for observed
emission lines and large infrared excess. To progress in the understanding of
the physical processes involved in the disk formation, we estimate the disk
parameters for a sample of Be stars and search for correlations between these
parameters and stellar properties. We performed spectro-interferometric
observations of 26 Be stars in the region of the Br$\gamma$ line to study the
kinematical properties of their disks through the Doppler effect. Observations
were performed at the Paranal observatory with the VLTI/AMBER interferometer.
This instrument provides high spectral and high spatial resolutions. We modeled
18 Be stars with emission in the Br$\gamma$ line. The disk kinematic is
described by a quasi-Keplerian rotation law, with the exception of HD28497 that
presents a one-arm density-wave structure. Using a combined sample, we derived
a mean value for the velocity ratio V/Vc=0.75, and found that rotation axes are
probably randomly distributed in the sky. Disk sizes in the line component
model are in the range of 2-13 stellar radii and do not correlate with the
effective temperature or spectral type. However, we found that the maximum size
of a stable disk correlates with the rotation velocity at the inner part of the
disk and the stellar mass. We found that, on average, the Be stars of our
combined sample do not rotate at their critical velocity. However, the
centrifugal force and mass of the star defines an upper limit size for a stable
disk configuration. For a given rotation, high-mass Be stars tend to have more
compact disks than their low-mass counterparts. It would be interesting to
follow up the evolution of the disk size in variable stars to better understand
the formation and dissipation processes of their circumstellar disks.
