  We present the first comprehensive thermal and rotational analysis of the
second most distant trans-Neptunian object (225088) 2007 OR10. We combined
optical light curves provided by the Kepler space telescope -- K2 extended
mission and thermal infrared data provided by the Herschel Space Observatory.
We found that (225088) 2007 OR10 is likely to be larger and darker than derived
by earlier studies: we obtained a diameter of d=1535^{+75}_{-225} km which
places (225088) 2007 OR10 in the biggest top three trans-Neptunian objects. The
corresponding visual geometric albedo is p_V=0.089^{+0.031}_{-0.009}. The light
curve analysis revealed a slow rotation rate of P_rot=44.81+/-0.37 h,
superseded by a very few objects only. The most likely light-curve solution is
double-peaked with a slight asymmetry, however, we cannot safely rule out the
possibility of having a rotation period of P_rot=22.40+/-0.18 h which
corresponds to a single-peaked solution. Due to the size and slow rotation, the
shape of the object should be a MacLaurin ellipsoid, so the light variation
should be caused by surface inhomogeneities. Its newly derived larger diameter
also implies larger surface gravity and a more likely retention of volatiles --
CH_4, CO and N_2 -- on the surface.
