I have similar interest. What will define the polygon? Input feature class? User draws on screen? The map view? What ArcGIS version are you using? From: http://help.arcgis.com/en/arcgisdesktop/10.0/help/index.html#//001w00000009000000 This Python code sets the environment extent (bounding box) that limits where geoprocessing tools operate:
import arcpy
from arcpy import env
# Set the extent environment using a keyword.
arcpy.env.extent = "MAXOF"
# Set the extent environment using the Extent class.
arcpy.env.extent = arcpy.Extent(-107.0, 38.0, -104.0, 40.0)
# Set the extent environment using a space-delimited string.
arcpy.env.extent = "-107.0 38.0 -104.0 40.0"
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