Currently rectangles are drawn from a center point to a corner. I find I pretty much always just click anywhere to get a rectangle, then edit the widthxheight to be what I want, then move it by picking it up by the corner.
Creating a rectangle corner-to-corner would make this much easier.
This is a frequent feature request, and one covered by my request that Carbide Create adhere to graphic design application standards (so alt click-drag-release would draw from the center, but click-drag-release would go corner-to-corner).