make a file in your home directory named .Xresources
mine has this in it (example to get you started)
Code:
rxvt*geometry: 110x40
rxvt*background: #010B45
rxvt*foreground: Wheat
rxvt*cursorColor: Orchid
rxvt*saveLines: 10000
rxvt*font: *fixed-bold-r-*-15-*-*
rxvt*scrollBar_right: True
any command line option you can use with rxvt can go in here
rxvt --help
to read in a new .Xresources for testing (happens automatically on login otherwise)
xrdb .Xresources