is there anyway I can change the size of the text that appears in the axis labels.
If you have one of the recent root version, you can use the provided user interface. After your drawing is created in the canvas, select the View menu / Editor. The root graphics editor will show up on the left of the canvas widow. Click with the left mouse button on the axis to select it. In the editor frame will appear the user interface for changing different axis attributes (color, size, offcet and font of labels, of axis title, etc.).
Any time you click on a different object in the canvas in this editor frame you will see the corresponding user interface that is provided for that object.
In your macro you can use any time:
axis->SetLabelSize(size); // Float_t size = 0.04 by default