ffmpeg size

I need to convert and Resize a video using ffmpeg (using ffmpeg on WINDOWS)
Works fine, except that resizing is FIXED size:
ex:
ffmpeg -i input.avi -s 320x240 output.flv

I am trying to have HALF the original size
ex:
ffmpeg -i input.avi -s 50% output.flv
And of course this does not wrok

I heard about midentify utility that returns the size of a video (this utility is supposed to be included into ffmpeg), but I cannot find it !
Where can I find this midentify utility for windows (which should solve my problem) or resize the video by half of its original size ?
(on windows only, no Linux utility)
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