fceux_config.txt

for some unknown reason, fceux uses base64 encoded values for window size multiplier (and aspect ratio too but that's less important)

anyways the important thing is this:

for 1x display (default unless you changed settings)
winsizemulx base64:AAAAAAAA8D8=
winsizemuly base64:AAAAAAAA8D8=

for 2x display
winsizemulx base64:AAAAAAAAAEA=
winsizemuly base64:AAAAAAAAAEA=

for 3x display
winsizemulx base64:AAAAAAAACEA=
winsizemuly base64:AAAAAAAACEA=