For horizontal resize, you need modify h front porch, hsync and h back porch on crt_range0 from mame.ini
crt_range0 15625-16200, 49.50-65.00, 2.000, 4.700, 8.000, 0.064, 0.192, 1.024, 0, 0, 192, 288, 448, 576
If you are in Windows, with crt emudriver you have a tool arcadeOSD that help you know these values.