I have a SHP that includes the entire county and want to downsize it to a specific area...how??
In qgis 1.8 you can use Vector->Geoprocessing-Tools -> clip:
Select Input vector (ex: AUT in screenshot ) clipping-polygon-Layer (ex: clipper in screenshot) and save clipped layer to disk (ex: GEclipt).
Attenion: copy prj file from input-file over clipped-layer-file, because this prj-file is empty. After this you can use / add it ...
Of course you can digitze your clipping-polygon layer in qgis, so you can use almost any shape
Using ArcGIS, there are two general methods for subsetting a shapefile. If you have features within your counties .shp (e.g. administrative voting areas, census zones etc), simply select those feature and export them as a new shapefile (Figure 1).
The second method involves clipping a subset of your county .shp based on a polygon. You can accomplish this using the Clip tool.