If you have the time and inclination, you can roll your own maps using mkgmap. This way you can just create a map for the very specific area you’re interested in which will create a smaller file. See my website for tips if you want to do this.

If not, you may be able to crop the map using GPSMapEdit. Freegeographytools has run a very good series of posts on apps for use with Garmin.
http://freegeographytools.com/2010/free-tools-for-custom-garmin-vector-maps-ii-gpsmapedit-and-gpsmapedit
and more generally
http://freegeographytools.com/2010/03 and http://freegeographytools.com/2010/04