Not at start-up. However, you can of course click the 'maximise' button at the top right of the Xterm window in order to use all the screen. Alternatively you can start up another window, for example by entering the following command in your existing Xterm window:
xterm -geom 132x50 &
(this opens a new Xterm window 132 characters wide by 50 deep)
You can vary the numbers 132 and 50 according to the Xterm window size required.
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