The effect of the geometrical confinement on the shell filling in quantum dots obtained in silicon-on-insulator (SOI) tri-sided gate metal oxide semiconductor single-electron transistors (MOSSETs) is investigated on the basis of the current spin density functional theory. Variations of the geometric shape in nanowire SOI-MOSSETs with square section entail important changes in the valley filling sequences, whereas the variability of the gate length does not imply a significant modification of the filling patterns. Our results provide quantitative insights towards the shell engineering of silicon quantum dots for qubits with stable valley filling patterns.