Code

Shape decision = Shape.fromId("Decision");
decision.setTextArea(Shape.fromId("Rectangle").getOutline());

shapeNode.setShape(decision);
shapeNode.setText("Could it be possible to have shapes which have text so long that it goes over the shape boundaries ?");
shapeNode.setTextFormat(new TextFormat(Align.Center, Align.Center));
shapeNode.getTextFormat().setWrapAtCharacter(false); 

Code

Shape decision = Shape.fromId("Decision");
decision.setTextArea(new ElementTemplate[] {
	new LineTemplate(-50, -50, 150, -50),
	new LineTemplate(150, -50, 150, 150),
	new LineTemplate(150, 150, -50, 150),
	new LineTemplate(-50, 150, -50, -50)
}); 

Code

// assuming rectangular shape
void fitText(ShapeNode node)
{
	Rectangle2D.Float nodeRect = node.getBounds();
	double nodeArea = nodeRect.width * nodeRect.height;
	Font font = node.getEffectiveFont();

	float paddingWidth = 2, paddingHeight = 2;
	Thickness padding = node.getTextPadding();
	if (padding != null)
	{
		paddingWidth = padding.getLeft() + padding.getRight();
		paddingHeight = padding.getTop() + padding.getBottom();
	}

	Dimension2D textSize = TextRenderer.measureText(
		node.getText(), nodeRect.width - paddingWidth, font);
	double textArea = textSize.getWidth() * (textSize.getHeight() + paddingHeight);
	while (textArea > nodeArea)
	{
		// due to changing line break positions, text area might not be strictly decreasing function?
		// make sure font size always decreases to avoid infinite loop
		double newSize = font.getSize2D() / Math.sqrt(textArea / nodeArea);
		newSize = Math.min(newSize, font.getSize2D() - 1);

		font = font.deriveFont((float)newSize);
		node.setFont(font);

		textSize = TextRenderer.measureText(
			node.getText(), nodeRect.width - paddingWidth, font);
		textArea = textSize.getWidth() * (textSize.getHeight() + paddingHeight);
	}
}