You can’t scale the rectangle with dimensions but there are other ways to get the rectangle to the desired size. You could draw it to the correct size to begin with entering dimensions as you would in SketchUp. You could add dimensions to the rectangle you’ve already drawn and move the edges until it’s the size you want. Or you could draw lines of the desired length parallel to the sides of the rectangle and use their endpoints as inference points when resizing the rectangle.
I could also measure the existing length, and calculate the scale factor to move the edge/corner the required precise distance. I tend to something similar inside SU to scale an inserted image.
I was thrown into confusion by the fact that Layout doesn’t seem to allow movement of an edge or corner by distance. It is otherwise such an intrinsic part of SU.
I think it might be possible to change position to another x-y position on the drawing (from the corner of the page), but that is inconvenient, and I haven’t tested it out.
I give up and will revert to one of the work-arounds. I had hoped that I might have missed something basic.
I have the unhappy knack of making these sorts of discoveries whenever doing something I haven’t tried before, while working on a project with a deadline. Then I have to find extra hours in the day to sort out the wrinkles before the project can continue.