how to define the area of grib

Hello I'm a new user. I can't figure out how to define the area for downloading a grib. I'm especially interested in the saildocs option as I want to use it with Iridium Go. The default seems to be a grib of the whole planet and I need a smaller file of the area of interest. On the Xgrib option the lat and long values cannot be changed either. Or am I missing something? 
Thanks! 

Hi,
Just select an area on the chart. Selection tool is the square icon in the toolbar. (on on Desktop version shift+drag).