[In Jm18] Wrap lines has an effect :

Depending on the number of labels and the available space, JMP arranges the labels into fewer lines and may even skip part of a label:
If you try to adjust the size of the labels to get graphs with consistent size, you could skip the fist step and use Frame Size :
Names Default to Here(1);
dt = Open( "$SAMPLE_DATA/Big Class.jmp" );
Graph Builder(
Variables( X( :height ), Y( :name ) ),
Elements( Bar( X, Y ) )
);
:name[2] = "This is a very long name which has an influence on the size of the Framebox";
current report()[FrameBox(1)] << Frame Size ( 500, 500)