  Direct angular size measurements of the G0IV subgiant $\eta$ Boo from the
Palomar Testbed Interferometer are presented, with limb-darkened angular size
of $\theta_{LD}= 2.1894^{+0.0055}_{-0.0140} $ mas, which indicate a linear
radius of $R=2.672 \pm 0.028 R_\odot$. A bolometric flux estimate of $F_{BOL} =
22.1 \pm 0.28\times 10^{-7}$ erg cm$^{-2}$s$^{-1}$ is computed, which indicates
an effective temperature of $T_{EFF}=6100 \pm 28$ K and luminosity of $L = 8.89
\pm 0.16 L_\odot$ for this object. Similar data are established for a check
star, HD 121860. The $\eta$ Boo results are compared to, and confirm, similar
parameters established by the {\it MOST} asteroseismology satellite. In
conjunction with the mass estimate from the {\it MOST} investigation, a surface
gravity of $\log g=3.817 \pm 0.016$ [cm s$^{-2}$] is established for $\eta$
Boo.
