Fused-filament fabrication, one of the most accessible additive manufacturing technology, has already been used to manufacture piezoresistive static/quasistatic strain sensors, force sensors and wind sensors. Recent research showed fused-filament fabrication could also be used for the manufacturing of dynamic sensors. However, the anisotropic mechanical and piezoresistive behaviour, the heterogeneity, the large number of process parameters and the large number of possible geometrical designs open up numerous design options for manufacturing. Additionally, a small size is difficult to achieve due to the relatively low geometrical tolerances and the resolution of fused-filament fabrication. This paper is focused on the design principles of a single-process, fused-filament fabricated, uni-axial accelerometer with a piezoresistive sensing element. The anisotropic piezoresistive behaviour is researched for the maximum sensitivity in the measured acceleration direction and the minimal cross-axis sensitivity for the perpendicular axes. The analytical accelerometer model is based on a Bernoulli-Euler beam and is used to understand the frequency and the geometrical influences of the piezoresistive accelerometer with a beam-shaped sensing element and an attached intertial mass. Finally, a small-sized (22x22x15 mm^3), single-axis, prototype accelerometer, with an approximately linear response up to-425 Hz and a low cross-axis sensitivity was manufactured. The derived principles can be used for further optimisation of the developed prototype sensor and as a foundation for the derivation of design principles for other types of fused-filament fabricated accelerometers, including 3-axis accelerometers.