The trick is to set the Window Settings the way you want it before you do anything in that Terminal window. Open Terminal, select "Window Settings" from the "Terminal" menu in the menubar, pull the drop-down menu to "Window" and set the size there. A common size is 80 colums x 24 rows. Then click the "Use Settings as Defaults" button at the bottom. Quit Terminal, re-open, and see if that worked.