This is an old post but I had the same issue and figured it out. You can set the width of a text edit box in a New Dialog window as shown in the following example.
win = New Window( "Example",
text = Text Edit Box( "Example Text" )
);
text << Set Width( 200 );
This is taken from JMP 9, Help > Displaybox Scripting > Text Edit Box > Set Width.
I put the width inside the dialog box as follows, which is a bit easier to follow if you have a lot of things in the dialog box:
win = New Window( "Example",
text = Text Edit Box( "Example Text", << Set Width( 200 ))
);
Regards,
Peter