How make a manual square?

I want a little square or plane, on bottom of my character to see his collision area.

But I don't know how to do.

Can anybody help me?

Here's some code I appropriated from some demo...add the following to CreateScene():

myLine = new Line3D();
myLine.addPoint(new Vector3(0, 0, 0));
myLine.addPoint(new Vector3(0, 0, 100));
myLine.addPoint(new Vector3(0, 100, 100));
myLine.addPoint(new Vector3(0, 100, 0));
myLine.addPoint(new Vector3(0, 0, 0));
myLine.drawLines();
SceneNode n = mSceneManager.GetRootSceneNode().CreateChildSceneNode("lines");
n.AttachObject(myLine);

MyLine is declared at the class level, as:

protected Line3D myLine;