A DESCRIPTION OF THE PROBLEM :
The -fx-cell-size property isn't working on table cells (.table-cell), it's only working on table rows (.table-row-cell). Both TableRowSkin and TableCellSkin inherit from TableCell, so this might be an intended behavior. However, in that case, there appears to be an error in Modena.

This rule in modena.css makes no difference. The resulting cell height is set only takes the padding and the font size into account.
https://github.com/openjdk/jfx/blob/master/modules/javafx.controls/src/main/resources/com/sun/javafx/scene/control/skin/modena/modena.css#L2395

.table-cell {
    -fx-padding: 0.166667em; /* 2px, plus border adds 1px */
    -fx-background-color: null;
    -fx-border-color: transparent -fx-table-cell-border-color transparent transparent;
    -fx-cell-size: 2.0em; /* 24 */
    -fx-text-fill: -fx-text-background-color;
}

STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
Run the attached code. Cell size should be 300px.

EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
If this is intended behavior:
* -fx-cell-size should be removed from the Modena CSS .table-cell rule.
* The scope of -fx-cell-size property should be clarified in the CSS Reference doc.

If this is not intended behavior, and -fx-cell-size is supposed to work on both table wows and table cells, then it should be fixed.

---------- BEGIN SOURCE ----------
import javafx.application.Application;
import javafx.beans.property.SimpleStringProperty;
import javafx.scene.Scene;
import javafx.scene.control.TableColumn;
import javafx.scene.control.TableView;
import javafx.stage.Stage;

public class Launcher extends Application {

    public static void main(String[] args) {
        launch();
    }

    @Override
    public void start(Stage stage) {
        var col = new TableColumn<String, String>("Foo");
        col.setCellValueFactory((o) -> new SimpleStringProperty(o.getValue()));

        var table = new TableView<String>();
        table.getColumns().add(col);
        table.getItems().addAll("foo", "bar", "baz");
        table.getStyleClass().add("my-table");

        var scene = new Scene(table, 800, 600);
        scene.getStylesheets().add(getClass().getResource("/index.css").toString());
        stage.setScene(scene);
        stage.show();
    }
}

index.css:
.my-table .table-cell {
    /* cell size isn't applied to the table cells */
    -fx-cell-size: 300px;
}
.my-table .table-row-cell {
   /* but it's still applied to rows */
    -fx-cell-size: 30px;
}
---------- END SOURCE ----------

FREQUENCY : always