For solving the Poisson equation in Fourier space, and for applying the local potential in Fourier space, an auxiliary cubic mesh is built. This mesh will be larger than the circumscribed cube of the usual mesh by a factor DoubleFFTParameter. See the section that refers to Poisson equation, and to the local potential for details [the default value of two is typically good].
poisson/poisson.F90 : 570
call parse_variable(namespace, 'DoubleFFTParameter', M_TWO, fft_alpha)