  The bright star 55 Cancri is known to host five planets, including a
transiting super-Earth. The study presented here yields directly determined
values for 55 Cnc's stellar astrophysical parameters based on improved
interferometry: $R=0.943 \pm 0.010 R_{\odot}$, $T_{\rm EFF} = 5196 \pm 24$ K.
We use isochrone fitting to determine 55 Cnc's age to be 10.2 $\pm$ 2.5 Gyr,
implying a stellar mass of $0.905 \pm 0.015 M_{\odot}$. Our analysis of the
location and extent of the system's habitable zone (0.67--1.32 AU) shows that
planet f, with period $\sim$ 260 days and $M \sin i = 0.155 M_{Jupiter}$,
spends the majority of the duration of its elliptical orbit in the
circumstellar habitable zone. Though planet f is too massive to harbor liquid
water on any planetary surface, we elaborate on the potential of alternative
low-mass objects in planet f's vicinity: a large moon, and a low-mass planet on
a dynamically stable orbit within the habitable zone. Finally, our direct value
for 55 Cancri's stellar radius allows for a model-independent calculation of
the physical diameter of the transiting super-Earth 55 Cnc e ($\sim 2.05 \pm
0.15 R_{\earth}$), which, depending on the planetary mass assumed, implies a
bulk density of 0.76 $\rho_{\earth}$ or 1.07 $\rho_{\earth}$.
