The following will allow you to change the font size of the x and y labels. I don't know how to change the legend font size though, it doesn't work the same way.
Open("$SAMPLE_DATA\Big Class.jmp");
Graph Builder(
Variables( X( :height ), Y( :age ) ),
Elements( Points( X, Y, Legend( 1 ), Jitter( 1 ) ), Smoother( X, Y, Legend( 2 ) ) ),
SendToReport( Dispatch( {}, "X title", TextEditBox, {Set Font Size( 20 )} ), ),
SendToReport( Dispatch( {}, "Y title", TextEditBox, {Set Font Size( 20 )} ), )
);