Of the order 10 meter.
The size of the Universe can be calculated by integrating the Friedman equation, which is a function of the densities of the components of the Universe (radiation, matter, dark energy, curvature, as well as more exotic components), as well as their equations of state. In general, there is no analytical result, but in certain epochs the history of the Universe, its dynamics are completely dominated by one or two of these components.
The early Universe was dominated by relativistic matter, i.e. radiation and neutrinos (early matter was also relativistic, but didn't contribute significantly to the energy density). In this case, integration yields the following relation between the scale factor $a$ (i.e. ratio between lengths at that time and today) and time $t$:
$$
a(t) \simeq \left( 2 \sqrt{\Omega_\mathrm{r,0}} H_0 t \right)^{1/2},
$$
where $\Omega_\mathrm{r,0}$ is today's value of the energy density of radiation relative to the critical density, and $H_0$ is the Hubble constant. For a Planck Collaboration et al. (2016) cosmology, at $t\sim10^{-32}\,\mathrm{s}$, this yields $2\times10^{-26}$.
That is, if inflation ended after $10^{-32}\,\mathrm{s}$, everything was $5\times10^{25}$ times closer to each other, or roughly 60 e-folding$^\dagger$.
The total Universe may or may not be infinite, but what we usually refer to when talking about the Universe, is the observable Universe, which is the part of the Universe from which light has had the time to reach us since Big Bang. The Universe is 13.8 Gyr old, but because it has expanded in the meantime, the observable Universe is more than 13.8 Glyr in radius — in fact $R_0 = 46.3\,\mathrm{Glyr}$.
Hence, the radius of what comprises "our" Universe today, was at time $t$ only $R(t) = a(t) R_0$, so at the end of inflation
$$
\begin{array}{rcl}
r(10^{-32}\,\mathrm{s}) & = & a(10^{-32}\,\mathrm{s}) \, R_0 \\
& = & 2\times10^{-26} \, \times \, 46.3\,\mathrm{Glyr} \\
& = & 9\,\mathrm{m}.
\end{array}
$$
If you think that inflation ended already after $10^{-33}\,\mathrm{s}$, you'll get $r=3\,\mathrm{m}$ instead.
$^\dagger$Coincidentally (I think) roughly the same number of e-foldings as inflation itself.