  We present the first rest-frame optical size-luminosity relation of galaxies
at $z>7$, using the NIRCam imaging data obtained by the GLASS James Webb Space
Telescope Early Release Science (GLASS-JWST-ERS) program, providing the deepest
extragalactic data of the ERS campaign. Our sample consist of 19
photometrically selected bright galaxies with $m_\text{F444W}\leq27.8$ at
$7<z<9$ and $m_\text{F444W}<28.2$ at $z\sim9-15$. We measure the size of the
galaxies in 5 bands, from the rest-frame optical ($\sim4800\,{\rm \AA}$) to the
ultra-violet (UV; $\sim1600\,{\rm \AA}$) based on the S\'ersic model, and
analyze the size-luminosity relation as a function of wavelength. Remarkably,
the data quality of NIRCam imaging is sufficient to probe the half-light radius
$r_e$ down to $\sim 100$ pc at $z>7$. Given the limited sample size and
magnitude range, we first fix the slope to that observed for larger samples in
rest-frame UV using HST samples. The median size $r_0$ at the reference
luminosity $M=-21$ decreases slightly from rest-frame optical ($600\pm80$ pc)
to UV ($450\pm130$ pc). We then re-fit the size-luminosity relation allowing
the slope to vary. The slope is consistent with $\beta\sim0.2$ for all bands
except F150W, where we find a marginally steeper slope of $\beta=0.53\pm0.15$.
The steep UV slope is mainly driven by the smallest and faintest galaxies. If
confirmed by larger samples, it implies that the UV size-luminosity relation
breaks toward the faint end as suggested by lensing studies.
