I assume you find the fact that the window starts so small annoying, and do not actually want to programatically change the window size.
The shortcut to maximize any window is
alt+F5. If that is too inconvenient you can change it via the system settings, or just drag the window up against the top edge of the screen, after making sure you have that activated in
settings -> window manager tweaks -> accessibility -> automatically tile windows when moving to screen edge
You can also change the default size of the terminal, which for
xfce4-terminal is done by selecting
edit -> preferences -> "appearance" tab -> Default geometry
This will not maximize your window by default though, just set it to the given size, so you might prefer the above solutions.