Check out the man page for xterm. You may have to add it to your MANPATH to get the page (should be /usr/openwin/man)

If the process is already running and the xterm is started by a program - you may not be able to change it.

The default lines in buffer is 64. You can change that with the -sl option.

/usr/openwin/bin/xterm -sl 100 would give you a xterm with 100 lines saved in the scroll buffer.