  The forthcoming generation of multi-petawatt lasers opens the way to abundant
pair production by the nonlinear Breit-Wheeler process, i.e., the decay of a
photon into an electron-positron pair inside an intense laser field. In this
paper we explore the optimal conditions for Breit-Wheeler pair production in
the head-on collision of a laser pulse with gamma photons. The role of the
laser peak intensity versus the focal spot size and shape is examined keeping a
constant laser energy to match experimental constraints. A simple model for the
soft-shower case, where most pairs originate from the decay of the initial
gamma photons, is derived. This approach provides us with a semi-analytical
model for more complex situations involving either Gaussian or Laguerre-Gauss
(LG) laser beams. We then explore the influence of the order of the LG beams on
pair creation. Finally we obtain the result that, above a given threshold, a
larger spot size (or a higher order in the case of LG laser beams) is more
favorable than a higher peak intensity. Our results match very well with
three-dimensional particle-in-cell simulations and can be used to guide
upcoming experimental campaigns.
