I'm using Raspberry Pi 3B. I will not go into details of RetroPie installation and configuration. There are much better resources on that.

Here is what I use to make RetroPie display image over composite video.

# If your TV doesn't have H-HOLD potentiometer then position of the left
# screen edge can be controlled with overscan_left setting.

overscan_left=16
#overscan_right=16
#overscan_top=16
#overscan_bottom=16

# composite PAL
sdtv_mode=2

# I'm using black and white TV, disabling colorburst is supposed
# to improve image quality on monochrome display,
# but I don't see any real difference
sdtv_disable_colourburst=1

# possible aspect ratios are 1=4:3, 2=14:9, 3=16:9; obviously this is 4:3 standard
sdtv_aspect=1

This way I get a 640x480 image with a glorious interlaced flicker of 50Hz refresh rate.