Hi Frank,
This display claims physical resolution 800x480, which I confirmed using OSMC with the similar setting in config.txt. The OSMC reports 800x480.
I tested with hdmi_cvt=640 480 60 6 0 0 0 in the config.txt, and got the same picture as I posted last time. If my display was 640x480, I would expect a fit full screen.
Here is my /boot/config.txt. I have a hifiberry DAC+ board with IR receiver plugged in.
#dtparam=i2c_arm=on
#dtparam=i2c_vc=on
dtparam=i2s=on
#dtparam=spi=on
#dtparam=act_led_trigger=mmc
#dtparam=uart1=off
#dtoverlay=pi3-disable-bt-overlay
# Uncomment one of these lines to enable an audio interface
#dtoverlay=hifiberry-dac
dtoverlay=hifiberry-dacplus
#dtoverlay=hifiberry-digi
#dtoverlay=hifiberry-amp
#dtoverlay=iqaudio-dac
#dtoverlay=iqaudio-dacplus
# Uncomment to enable the lirc-rpi module
# Params: gpio_out_pin GPIO pin for output (default "17")
# gpio_in_pin GPIO pin for input (default "18")
# gpio_in_pull Pull up/down/off on the input pin
# (default "down")
# sense Override the IR receive auto-detection logic:
# "1" = force active high
# "0" = force active low
# "-1" = use auto-detection
# (default "-1")
# softcarrier Turn the software carrier "on" or "off"
# (default "on")
# invert "on" = invert the output pin (default "off")
# debug "on" = enable additional debug messages
# (default "off")
dtoverlay=lirc-rpi
#dtparam=gpio_out_pin=17
dtparam=gpio_in_pin=26
#dtparam=gpio_in_pull=down
# LCD settings
# 180 degrees
lcd_rotate=2
# Normal
#lcd_rotate=0
#dtoverlay=rpi-backlight
#dtoverlay=rpi-ft5406
# Uncomment if you get no picture on HDMI for a default "safe" mode
#hdmi_safe=1
# Uncomment this if your display has a black border of unused pixels visible
# and your display can output without overscan
#disable_overscan=1
# Uncomment the following to adjust overscan. Use positive numbers if console
# goes off screen, and negative if there is too much border
#overscan_left=-32
#overscan_right=16
#overscan_top=-32
#overscan_bottom=16
# Uncomment to force a console size. By default it will be display's size minus
# overscan.
#framebuffer_width=800
#framebuffer_height=480
# Uncomment if hdmi display is not detected and composite is being output
#hdmi_force_hotplug=1
#Added to use HDMI output 800x480
#hdmi_force_hotplug=1
hdmi_group=2
hdmi_mode=87
hdmi_cvt=800 480 60 6 0 0 0
# Uncomment to force a specific HDMI mode (this will force VGA)
#hdmi_group=1
#hdmi_mode=1
# Uncomment to force a HDMI mode rather than DVI. This can make audio work in
# DMT (computer monitor) modes
hdmi_drive=2
# Uncomment to set monitor mode to DMT
#hdmi_group=2
# Uncomment to increase signal to HDMI, if you have interference, blanking, or
# no display
#config_hdmi_boost=4
# setting the max usb current to 1200mA
max_usb_current=1
# memory options
gpu_mem=64
# Overclocking options
force_turbo=1