I can unterstand why because the integration over Grassman variables has to be translational invariant too, one has
$$ \int d\theta = 0 $$
and
$$ \int d\theta \theta = 1 $$
but I dont see where the rule for this double integration
$$ \int d^2 \theta \bar{\theta}\theta = -2i $$
comes from.
So can somebody explain to me how this is motivated and/or derived?