I assume your real question is: can i modify a shape after you already drew it.
-select the shape
-edit->free transform (or "ctrl+T")
-in the top side of the screen , in the transform bar you will notice the options appear X:value(px) Y:value(px) W:value(%) and H:value(%) and some other regarding agles.
-modify the W: and the H: value by inputing the size you want.Even though the values are given in "%" by default, just type your own value and add "px" or any other type of unit(works in cm, inches, etc).
Ex: if i want to modify a rectangle to have the size of 800/600px i just modify the W:100% to W:800px and the H:100% to H:600px
If you play a bit with that bar you will discover how to position too,easier to draw guides though