So first your going to open up the Autodesk Inventor Software. Select the "New" icon in the top left hand corner of the inventor screen. Then select the standard.ipt icon in the screen that pops up. Once the screen is finished loading you're going to select the rectangle icon in the top left hand corner of the screen. Click once at any point on the screen and then click once more on the opposite side above or below your original point to create a rectangle. The rectangle should be green now. It doesn't matter what size your rectangle is, we are going to change the dimensions anyway.
Step 2: Orient your object to the Origin
Now we are going to change the orientation of our rectangle. Start by selecting the horizontal constraint button at the top of the screen. Now click on the green midpoint of the vertical leg of the rectangle. Then click on the origin point. The object will now be horizontally centered in relation to the origin. Now select the vertical constraint button on the top of the screen. Select the midpoint of the top line and then click the origin point as we did before. Your object will now be perfectly centered around the origin.
Step 3: Resize You Object to Desired Dimension
Now we are going to dimension our object to the size we want. Our rectangle is going to be 4.5" x 2.5". Select the dimension icon at the top of the screen and then move your cursor onto the top line of the rectangle. It will turn white. Click on it move the cursor up to place your dimension line. Enter the desired size into the box and click the green check. Repeat this for the vertical line as well. You should end up with a blue rectangle 4.5" x 2.5".