How to get a file's EXTENT values?

The easiest way to get a vector file's EXTENT is to use the ogrinfo utility, that is part of the GDAL/OGR library (for raster files you would use the gdalinfo utility). Windows users can download the FWTools package, which includes all of the GDAL and OGR commandline utilities. MS4W also includes the utilities (in ms4w/tools/gdal-ogr-utils/). Linux users will probably already have the GDAL libraries, if not you can also use the FWTools package.

For example, here is the results of the ogrinfo command on a shapefile (notice the "Extent" line):

$ ogrinfo province.shp province -summary
  INFO: Open of `province.shp'
  using driver `ESRI Shapefile' successful.

  Layer name: province
  Geometry: Polygon
  Feature Count: 1071
  Extent: (-2340603.750000, -719746.062500) - (3009430.500000, 3836605.250000)
  Layer SRS WKT:
  (unknown)
  AREA: Real (16.0)
  PERIMETER: Real (16.0)
  PROVINCE_: Real (16.0)
  PROVINCE_I: Real (16.0)
  STATUS: String (64.0)
  NAME: String (64.0)
  NAME_E: String (64.0)
  NAME_F: String (64.0)
  REG_CODE: Real (16.0)
  POLY_FEATU: Real (16.0)
  ISLAND: String (64.0)
  ISLAND_E: String (64.0)
  ISLAND_F: String (64.0)
  YYY: Real (16.0)
  SIZE: Real (16.0)
  ANGLE: Real (16.0)

Ogrinfo gives the file's extent in the form of (minx, miny),(maxx, maxy), therefore the EXTENT in a mapfile would be:

EXTENT -2340603.750000 -719746.062500 3009430.500000 3836605.250000

Note that the EXTENT in a mapfile must be in the same units as the MAP-level PROJECTION.