I'd been looking to replace my use of terminal.app with X11/xterm, since I often find my self running X11 apps on remote hosts. But I couldn't find a preference to set the xterm dimensions -- I'm used to a minimum of 132 columns and manually resizing the xterm window at each launch finally drove me to find out how to do it. Launch X11, then select Applications -> Customize. The X11 Application Menu should have an entry something like:
Terminal | xterm | n
Double-click the xterm command field and simply add the
-geometry parameter. The following will set 132 columns x 50 rows for the initial and subsequent xterms:
xterm -geometry 132x50
You can read about other parameters for xterm in the man page from within xterm itself via
man xterm.