  Magnetic reconnection in the relativistic regime has been proposed as an
important process for the efficient production of nonthermal particles and
high-energy emissions. Using fully kinetic particle-in-cell simulations, we
investigate how guide-field strength and domain size affect characteristic
spectral features and acceleration processes. We study two stages of
acceleration: energization up until the injection energy $\gamma_{\rm inj}$ and
further acceleration that generates a power-law spectrum. Stronger guide fields
increase the power-law index and $\gamma_{\rm inj}$, which suppresses
acceleration efficiency. These quantities seemingly converge with increasing
domain size, suggesting that our findings can be extended to large-scale
systems. We find that three distinct mechanisms contribute to acceleration
during injection: particle streaming along the parallel electric field, Fermi
reflection, and the pickup process. Fermi and pickup processes, related to the
electric field perpendicular to the magnetic field, govern the injection for
weak guide fields and larger domains. Meanwhile, parallel electric fields are
important for injection in the strong guide field regime. In the post-injection
stage, we find that perpendicular electric fields dominate particle
acceleration in the weak guide field regime, whereas parallel electric fields
control acceleration for strong guide fields. These findings will help explain
the nonthermal acceleration and emissions in high-energy astrophysics,
including black hole jets and pulsar wind nebulae.
