  Let $\Phi$ be a classical root system and $k$ be a field of sufficiently
large characteristic. Let $G$ be the classical group over $k$ with the root
system $\Phi$, $U$ be its maximal unipotent subgroup and $\mathfrak{u}$ be the
Lie algebra of $U$. Let $D$ be an orthogonal subset of $\Phi$ and $\Omega$ be a
coadjoint orbit of $U$ associated with $D$. We construct a polarization of
$\mathfrak{u}$ at the canonical form on $\Omega$. We also find the dimension of
$\Omega$ in terms of the Weyl group of $\Phi$. As a corollary, we determine all
possible dimensions of irreducible complex represenations of the group $U$ for
the case of finite field $k$.
