  We study the size of large-scale motions in turbulent plane Couette flows at
moderate Reynolds number up to $Re_\tau$ = 500. Direct numerical simulation
domains were as large as $100\pi\delta\times2\delta\times5\pi\delta$, where
$\delta$ is half the distance between the walls. The results indicate that
there are structures with streamwise extent, as measured by the wavelength, as
long as 78$\delta$ and at least 310$\delta$ at $Re_\tau$ = 220 and 500,
respectively. The presence of these very long structures is apparent in the
spectra of all three velocity components and the Reynolds stress. In DNS using
a smaller domain, the large structures are constrained, eliminating the
streamwise variations present in the larger domain. Effects of a smaller domain
are also present in the mean velocity and the streamwise velocity variance in
the outer flow.
