Match the documentation of -s with the current implementation.

doc/ffmpeg.texi

@@ -168,7 +168,10 @@ Set the number of video frames to record.
 @item -r @var{fps}
 Set frame rate (Hz value, fraction or abbreviation), (default = 25).
 @item -s @var{size}
-Set frame size. The format is @samp{wxh} (ffserver default = 160x128, ffmpeg default = same as source).
+Set frame size. The format is @samp{wxh} (ffserver default = 160x128).
+There is no default for input streams,
+for output streams it is set by default to the size of the source stream.
+If the input file has video streams with different resolutions, the behaviour is undefined.
 The following abbreviations are recognized:
 @table @samp
 @item sqcif