Try this:

my.eflags |= FAT | NARROW; // set both flags to prevent automatic recalculation on scale changes
vec_set(my.min_x,vector(-10,-10,-25)); // set bounding box to individual values
vec_set(my.max_x,vector(10,10,25));

where the smaller numbers are the width and breadth and the longer, height.

Play with the numbers and see the results