Whenever you launch the xterm the font size will be very small so to increase it here is some command line parameter that you can pass to increase the font size.
username$ xterm -geometry 72×34+100+40 -fn *-fixed-*-*-*-20-*
The above command can be used directly from the command line
But will we remember the above command all the time. surely I will not. so I added the following entry to the ~/.bashrc file
alias xterm=”xterm -geometry 72×34+100+40 -fn *-fixed-*-*-*-20-*”
now I can run xterm it will now have larger font by default.