Hi Davide,
You can use g.region zoom=filename, where the filename is the name
of a binary raster map ( you r.reclass to create this binary map where the ONES
are the data with the information you wish to use. This is essentially like
using a mask. What g.region zoom= does is set your region to be the smallest
possible area that will encompass all the data (the non zero data). If you
need to make the region a little bit bigger use g.region n=n+xxx s=s-xxx etc..
basically check out the g.region command in the manual...
Don Catanzaro