Width - set the percentage from the centre of the comp to align your object to the width of the composition.
Height - set the percentage from the centre of the comp to align your object to the height of the composition.
Offset - add/subtract a value (px) to the width and height settings.
In combination with Dynamic Rendering this can be used to output several sizes of the same Composition. In the example below we've connected the Comp's Width and Height to a couple of Value Arrays and then connected the Dynamic Index from the Render Manager to the Array Index of the Value Array. We've also used the same technique to set the Text Shape's Size.