You need to install the wmctrl package. Then, add this to your ~/.bashrc file:

HORIZONTAL=600

VERTICAL=400

wmctrl -r ":ACTIVE:" -e 0,-1,-1,${HORIZONTAL},${VERTICAL}

It will resize your active window (should be the terminal), to the size specified by the ${HORIZONTAL} and ${VERTICAL} variables. Note that these are measured in pixels. You can set your terminal's geometry to what you want, then use the screenruler package to measure the dimensions in pixels.

Hope this helps :)

EDIT: Actually you can run this to show the size of all the gnome-terminal's that are open:

wmctrl -l -x -G | grep gnome-terminal.Gnome-terminal | awk '{ print "Horizontal: "$5" pixels; Vertical: "$6" pixels" }'