All transform operations in AE are relative to the anchor point, which has its own keyframable property. There is no free transform like in Photoshop for this stuff (there is for masks and shape layer paths, though). In any case, I would advise not to use that methodology, anyway. Scaling solids will just like any other layer resample them, which you will notice when applying effects at the latest. Instead use Layer --> Layer Settings to resize the solid's source and keep it clean and then create other sized solids for your other stuff when needed...
Ah, perfect. It's a Mask on top of the layer solid that I need to create to do what I want. You've answered my question and concern, thank you.
Since you're new to After Effects, I strongly rcommend that you take some time to work your way through the basic learning materials.
Here is a good place to start to learn After Effects: http://adobe.ly/bjBT3P