  For terahertz spectroscopy on single crystals, the wavelength $\lambda$ often
is comparable to the size of the studied samples, emphasizing diffraction
effects. Using a continuous-wave terahertz spectrometer in transmission
geometry, we address the effect of the sample size on the achievable accuracy
of the optical properties, focusing in particular on the phase data. We employ
$\alpha$-lactose monohydrate as a paradigmatic example and compare data that
were measured using apertures with diameters D in the range from 10 mm to 0.2
mm. For small D, strong diffraction typically invalidates a quantitative
analysis of the transmitted amplitude at low frequencies. The phase data,
however, can be evaluated to lower frequency and show a more systematic
dependence on D. For a quantitative analysis, we employ a waveguide picture for
the description of small apertures with a cylindrical bore. For D as small as
0.2 mm, corresponding to 1/D = 50 cm$^{-1}$, a circular waveguide does not
support propagating waves below its cut-off frequency $1/\lambda_c =
\omega_c/2\pi c \approx 29$ cm$^{-1}$. Experimentally, we confirm this cut-off
for cylindrical apertures with a thickness $d_{ap} = 1$ mm. Close to
$\omega_c$, the measured phase velocity is an order of magnitude larger than
$c$, the speed of light in vacuum. The cut-off is washed out if a sample is
mounted on a thin aperture with a conical bore. In this case, the phase data of
$\alpha$-lactose monohydrate for D = 0.2 mm can quantitatively be described
down to about 10 cm$^{-1}$ if the waveguide-like properties of the aperture are
taken into account in the analysis.
