In your css file you can add
.tree-cell .label {
-fx-wrap-text: true;
}
You can find the css reference here http://docs.oracle.com/javafx/2/api/javafx/scene/doc-files/cssref.html
You can see the current settings by looking at caspian.css in your jfxrt.jar file or just using google to find it on the internet.
You can also use .setCellFactory to specify how the cells are made. I use TextFields for editing but I just changed it to TextArea and that works fine as well, except for a little problem with pressing enter but that's easy to fix.