there is a configuration file .xawtv (note the period, it is a hidden file).
.xawtv is in /root or /home/user directory

This is a fragment. Note the line fullscreen=640x480

[global]
freqtab = us-cable
pixsize = 128 x 96
pixcols = 1
jpeg-quality = 75
mjpeg-quality = 50
keypad-ntsc = yes
osd = yes
mixer = line
fullscreen =640x480