  We present updated parameters for the star HD 209458 and its transiting giant
planet. The stellar angular diameter $\theta$=0.2254$\pm$0.0017 mas is obtained
from the average ratio between the absolute flux observed with the Hubble Space
Telescope and that of the best-fitting Kurucz model atmosphere. This angular
diameter represents an improvement in precision of more than four times
compared to available interferometric determinations. The stellar radius
$R_\star$=1.20$\pm$0.05 R$_{\odot}$ is ascertained by combining the angular
diameter with the Hipparcos trigonometric parallax, which is the main
contributor to its uncertainty, and therefore the radius accuracy should be
significantly improved with Gaia's measurements. The radius of the exoplanet
$R_\text{p}$=1.41$\pm$0.06 $R_\text{J}$ is derived from the corresponding
transit depth in the light curve and our stellar radius. From the model
fitting, we accurately determine the effective temperature,
$T_\text{eff}$=6071$\pm$20 K, which is in perfect agreement with the value of
6070$\pm$24 K calculated from the angular diameter and the integrated spectral
energy distribution. We also find precise values from recent Padova Isochrones,
such as $R_\star$=1.20$\pm$0.06 R$_{\odot}$ and $T_\text{eff}$=6099$\pm$41 K.
We arrive at a consistent picture from these methods and compare the results
with those from the literature.
