Increase the font size of xterm

nano ~/.Xresources

Syntax is:

xterm*faceName: <font-name>:size=<font-size>

Now add (type/paste) the following line:

xterm*faceName: Monospace:size=15

Save the file.

Now run the following command:

xrdb -merge ~/.Xresources

Thats it.

Enjoy :-)