Place this in your ```~/.bash_aliases``` file.

zoom_xfce4_terminal.sh

zi() {
        if [[ ! -z $1 ]]; then
                steps=$1;
                SIZE=`grep 'FontName' ~/.config/Terminal/terminalrc | cut -d' ' -f 2`
                NEWSIZE=$((SIZE + steps))
                REGEXPR='s/FontName.*/FontName=Monospace '$NEWSIZE'/g'
                sed -i "$REGEXPR" ~/.config/Terminal/terminalrc
        else
                echo "Usage: zi [zoom steps]";
        fi;
}
zo() {
        if [[ ! -z $1 ]]; then
                steps=$1;
                SIZE=`grep 'FontName' ~/.config/Terminal/terminalrc | cut -d' ' -f 2`
                NEWSIZE=$((SIZE - steps))
                REGEXPR='s/FontName.*/FontName=Monospace '$NEWSIZE'/g'
                sed -i "$REGEXPR" ~/.config/Terminal/terminalrc
        else
                echo "Usage: zi [zoom steps]";
        fi;
}
z() {
        if [[ ! -z $1 ]]; then
                SIZE=$1;
                REGEXPR='s/FontName.*/FontName=Monospace '$SIZE'/g'
                sed -i "$REGEXPR" ~/.config/Terminal/terminalrc
        else
                echo "Usage: z [font size]"
        fi;
}

A slight tweak to the underlying code that allows for the font name to be something other than Monospace ... (with a fixed step of -2 in this case, but that's obviously easy to adapt)

#!/bin/bash
CONFIG=~/.config/xfce4/terminal/terminalrc
SIZE=`grep 'FontName' $CONFIG | awk '{print $NF}'`
NEWSIZE=$((SIZE - 2))
## echo "old size $SIZE, new size $NEWSIZE"
REGEXPR='s/FontName\(.*\)'$SIZE'$/FontName\1'$NEWSIZE'/g'
sed -i "$REGEXPR" $CONFIG