You can try to adjust them with << Set Sizes
Names Default To Here(1);
dt = Open("$SAMPLE_DATA/Big Class.jmp");
w_dt = Get Window(dt);
sb_dt = w_dt[SplitterBox(1)];
Show(sb_dt < get sizes);
wait(1); // demo purposes
w_dt[SplitterBox(1)] << Set Sizes({0.6, 0.3, 0.1})
and you can use << get sizes to get the initial sizing if needed
-Jarmo