How can one scale both a rectangle and a circle--both being selected--so that the circle is scaled appropriately according to what I want to specify the rectangle's width to be?
For precision, if you need the objects to be a specific width....
If you want to scale by a percentage.
You can also perform math functions in dimension fields. (+ plus, - minus, * multiply, and / divide) So if you know your square is 100pt wide and you want the circle to be 80% of 100pts, you could input
100*.8 into the Width field (100pts times .8 = 80% of 100):
Hit enter and the value will be calculated (80pts)