From the man ffmpeg page these are the enties that go with the -s option to set your screen size. Use the abbreviation, not the actual numbers.
-s size Set frame size. The format is wxh (avserver default = 160x128,
ffmpeg default = same as source). The following abbreviations are
recognized:
-s size
sqcif
128x96
qcif
176x144
cif 352x288
4cif
704x576
16cif
1408x1152
qqvga
160x120
qvga
320x240
vga 640x480
svga
800x600
xga 1024x768
uxga
1600x1200
qxga
2048x1536
sxga
1280x1024
qsxga
2560x2048
hsxga
5120x4096
wvga
852x480
wxga
1366x768
wsxga
1600x1024
wuxga
1920x1200
woxga
2560x1600
wqsxga
3200x2048
wquxga
3840x2400
whsxga
6400x4096
whuxga
7680x4800
cga 320x200
ega 640x350
hd480
852x480
hd720
1280x720
hd1080
1920x1080
-s size Set frame size. The format is wxh (avserver default = 160x128,
ffmpeg default = same as source). The following abbreviations are
recognized:
-s size
sqcif
128x96
qcif
176x144
cif 352x288
4cif
704x576
16cif
1408x1152
qqvga
160x120
qvga
320x240
vga 640x480
svga
800x600
xga 1024x768
uxga
1600x1200
qxga
2048x1536
sxga
1280x1024
qsxga
2560x2048
hsxga
5120x4096
wvga
852x480
wxga
1366x768
wsxga
1600x1024
wuxga
1920x1200
woxga
2560x1600
wqsxga
3200x2048
wquxga
3840x2400
whsxga
6400x4096
whuxga
7680x4800
cga 320x200
ega 640x350
hd480
852x480
hd720
1280x720
hd1080
1920x1080
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