  Using particle-in-cell (PIC) numerical simulations with electron-positron
pair plasma, we study how the efficiencies of magnetic dissipation and particle
acceleration scale with the initial coherence length $\lambda_0$ in relation to
the system size $L$ of the two-dimensional (2D) `Arnold-Beltrami-Childress'
(ABC) magnetic field configurations. Topological constraints on the
distribution of magnetic helicity in 2D systems, identified earlier in
relativistic force-free (FF) simulations, that prevent the high-$(L/\lambda_0)$
configurations from reaching the Taylor state, limit the magnetic dissipation
efficiency to about $\epsilon_{\rm diss} \simeq 60\%$. We find that the peak
growth time scale of the electric energy $\tau_{\rm E,peak}$ scales with the
characteristic value of initial Alfven velocity $\beta_{\rm A,ini}$ like
$\tau_{\rm E,peak} \propto (\lambda_0/L)\beta_{\rm A,ini}^{-3}$. The particle
energy change is decomposed into non-thermal and thermal parts, with
non-thermal energy gain dominant only for high initial magnetisation. The most
robust description of the non-thermal high-energy part of the particle
distribution is that the power-law index is a linear function of the initial
magnetic energy fraction.
