Some pre-configurations of output dimensions on ffmpeg.

ffmpeg -i source.ext -r 1 -s 4cif -f image2 img%5d.jpg

This will produce you pictures with 704 width * 576 height

sqcif	128×96	qcif	176×144	cif	352×288
4cif	704×576	qqvga	160×120	qvga	320×240
vga	640×480	svga	800×600	xga	1024×768
uxga	1600×1200	qxga	2048×1536	sxga	1280×1024
qsxga	2560×2048	hsxga	5120×4096	wvga	852×480
wxga	1366×768	wsxga	1600×1024	wuxga	1920×1200
woxga	2560×1600	wqsxga	3200×2048	wquxga	3840×2400
whsxga	6400×4096	whuxga	7680×4800	cga	320×200
hd480	852×480	hd720	1280×720	hd1080	1920×1080