See this page by @djmannion where he describes how to set the spatial frequency if the stimulus has dimensions defined in pixels:
http://www.djmannion.net/psych_programming/vision/draw_gratings/draw_gratings.html#spatial-frequency
and an old post from @jon here:
https://groups.google.com/d/msg/psychopy-users/KrMtf6fWZu0/Vl6EI3MN9OMJ