The only way I have found is to reduce the Text Font size in preferences. However that will also change the legend font size and the wrap heading accordingly.
It can be somewhat automated with jsl (partly reused code from a recent similar thread):
wrap_column = Column( "room/office" );
dt = Open( "$sample_data\S4 Temps.jmp" );
Graph Builder(
Show Control Panel( 0 ),
Variables( X( :outside temp ), Y( :Y ), Wrap( wrap_column ) ),
Elements(
Points( X, Y, Legend( 1 ) ),
Line( X, Y, Legend( 3 ), Row order( 0 ), Summary Statistic( "Mean" ) )
),
SendToReport( Dispatch( {}, "outside temp", ScaleBox, {Rotated Labels( "Automatic" )} ) )
);
// Change Text Font size
p_expr = Expr( Get Preferences( text font ) );
p_start = p_expr;
scale_factor = 20; // Adjust according to string length and one's own liking
Summarize( g = by( wrap_column ) );
For( i = 1, i <= N Items( g ) / scale_factor, i++,
p = p_expr;
Preferences( text font( Arg( Arg( p, 1 ), 1 ), Arg( Arg( p, 1 ), 2 ) - 1 ) );
);
// Uncomment and run the code below to reset original setting after export of graph
// Eval( p_start );