  We consider random walk and self-avoiding walk whose 1-step distribution is
given by $D$, and oriented percolation whose bond-occupation probability is
proportional to $D$. Suppose that $D(x)$ decays as $|x|^{-d-\alpha}$ with
$\alpha>0$. For random walk in any dimension $d$ and for self-avoiding walk and
critical/subcritical oriented percolation above the common upper-critical
dimension $d_{\mathrm{c}}\equiv2(\alpha\wedge2)$, we prove large-$t$
asymptotics of the gyration radius, which is the average end-to-end distance of
random walk/self-avoiding walk of length $t$ or the average spatial size of an
oriented percolation cluster at time $t$. This proves the conjecture for
long-range self-avoiding walk in [Ann. Inst. H. Poincar\'{e} Probab. Statist.
(2010), to appear] and for long-range oriented percolation in [Probab. Theory
Related Fields 142 (2008) 151--188] and [Probab. Theory Related Fields 145
(2009) 435--458].
