If you use XTerm and want to control window size and window position, use -geometry option.
xterm -geometry 150x50+0+0 &
Will open an XTerm 150 chars wide, 50 chars high in the top left hand corner of the screen
Another way is write some lines to file .Xresources in user home directory.
xterm*geometry: 100x40+0+0 XTerm*faceName: DejaVu Sans Mono XTerm*faceSize: 14
Last two strings setup font face and font size for xterm.
The final step is to set these resource changes using xrdb
xrdb -merge .Xresources