  This study aims to determine the size, albedo and rotational period of
(98943) 2001 CC21, target of the Hayabusa2 extended mission, using thermal data
from the Spitzer Space telescope and ground based observations. The Spitzer
data were acquired with the Infrared Spectrograph in the 6-38 micron range,
reduced using the Spitzer pipeline and modeled with the Near Earth Asteroid
Thermal Modeling to determine the asteroid size and albedo. The absolute
magnitude and the rotational period were determined thanks to new observations
carried out at the 3.5m New Technology Telescope, at the 1.2m Observatoire de
Haute Provence, and at the 0.7m Abastumani telescope. Three complete
lightcurves were obtained in 2023-2024 at the last mentioned telescope. We
determine an absolute magnitude of H=18.94$\pm$0.05, and a rotational period of
5.02124$\pm$0.00001 hours, with a large lightcurve amplitude of $\sim$ 0.8 mag.
at a phase angle of 22$^o$, indicating a very elongated shape with estimated
a/b semiaxis ratio $\geq$ 1.7, or a close-contact binary body. The emissivity
of 2001 CC21 is consistent with that of silicates, and its albedo is
21.6$\pm$1.6 %. Finally, the spherical-equivalent diameter of 2001 CC21 is
465$\pm$15 m. The albedo value and emissivity here determined, coupled with
results from polarimetry and spectroscopy from the literature, confirm that
2001 CC21 is an S-complex asteroid, and not a L-type, as previously suggested.
The size of 2001 CC21 is less than 500 m, which is smaller than its first size
estimation ($\sim$ 700 m). These results are relevant in preparation of the
observing strategy of 2001 CC21 by Hayabusa2 extended mission.
