  With three-dimensional hydrodynamical simulations we show that the size of
the decretion disc and the structure of the accretion flow onto the neutron
star in a Be/X-ray binary strongly depends upon the disc aspect ratio, $H/R$.
We simulate a Be star disc that is coplanar to the orbit of a circularly or
moderately eccentric neutron star companion, thereby maximising the effects of
tidal truncation. For low disc aspect ratio, $H/R\lesssim 0.1$, the disc is
efficiently tidally truncated by the neutron star. Most material that escapes
the Roche lobe of the Be star is accreted by the neutron star through tidal
streams. For larger disc aspect ratio, the outflow rate through the Be star
disc is higher, tidal truncation becomes inefficient, the disc fills the Roche
lobe and extends to the orbit of the companion. Some material escapes the
binary as a gas stream that begins near the L2 point. While the accretion rate
onto the neutron star is higher, the fraction of the outflow that is accreted
by the neutron star is smaller. Low density Be star discs are expected to be
approximately isothermal, such that $H/R$ increases with radius. Tidal
truncation is therefore weaker for larger separation binaries, and lower mass
primaries.
