In the moduli space of polarized abelian varieties in positive characteristic we define leaves. The dimension of such a leaf only depends on the related Newton polygon. We give a formula for this dimension. We prove this to be the correct formula in two different ways. The proofs depend on the notion of "minimal p-divisible groups" (FO), a result by Wedhorn computing dimensions of EO-strata, and on a generalization by Chai of Serre-Tate coordinates.