  The disk of FU Orionis is marginally resolved with MATISSE, suggesting that
the region emitting in the thermal infrared is rather compact. An upper limit
of $\sim1.3\pm0.1$ mas (in $L$) can be given for the diameter of the disk
region probed in the $L$ band, corresponding to 0.5 au at the adopted Gaia EDR3
distance. This represents the hot, gaseous region of the accretion disk. The
$N$-band data indicate that the dusty passive disk is silicate-rich. Only the
innermost region of said dusty disk is found to emit strongly in the $N$ band,
and it is resolved at an angular size of $\sim5$ mas, which translates to a
diameter of about 2 au. The observations therefore place stringent constraints
for the outer radius of the inner accretion disk. Dust radiative transfer
simulations with RADMC-3D provide adequate fits to the spectral energy
distribution from the optical to the submillimeter and to the interferometric
observables when opting for an accretion rate $\dot{M}\sim 2\times 10^{-5}\,
M_\odot$ yr$^{-1}$ and assuming $M_*=0.6\, M_\odot$. Most importantly, the hot
inner accretion disk's outer radius can be fixed at 0.3 au. The outer radius of
the dusty disk is placed at 100 au, based on constraints from scattered-light
images in the literature. The dust mass contained in the disk is
$2.4\times10^{-4}\, M_\odot$, and for a typical gas-to-dust ratio of 100, the
total mass in the disk is approximately 0.02 $M_\odot$. We did not find any
evidence for a nearby companion in the current interferometric data, and we
tentatively explored the case of disk misalignment. For the latter, our
modeling results suggest that the disk orientation is similar to that found in
previous imaging studies by ALMA. Should there be an asymmetry in the very
compact, inner accretion disk, this might be resolved at even smaller spatial
scales ($\leq1$ mas).
