  We present calculations of the electronic and thermal transport properties of
graphene antidot lattices with a finite length along the transport direction.
The calculations are based on a single orbital tight-binding model and the
Brenner potential. We show that both electronic and thermal transport
properties converge fast toward the bulk limit with increasing length of the
lattice: only a few repetitions (~6) of the fundamental unit cell are required
to recover the electronic band gap of the infinite lattice as a transport gap
for the finite lattice. We investigate how different antidot shapes and sizes
affect the thermoelectric properties. The resulting thermoelectric figure of
merit, ZT, can exceed 0.25, and it is highly sensitive to the atomic
arrangement of the antidot edges. Specifically, hexagonal holes with pure
zigzag edges lead to an order-of-magnitude smaller ZT as compared to pure
armchair edges. We explain this behavior as a consequence of the localization
of states, which predominantly occurs for zigzag edges, and of an increased
splitting of the electronic minibands, which reduces the power factor.
