The transform toolbar (shown below) can be used to specify the exact position and dimensions for paths/shapes and selections. I will explain how to use the toolbar. Once you understand how to use the toolbar you task is quite simple.

The toolbar above can be read like this: Make my object 10px wide and 200px tall, then place its top-left corner on the top edge of the canvas, 10px right of the left edge.

Sizing

The red arrows show where the new size of your object can be entered. By default, the value is shown as 100% and any plain number that is entered is automatically converted to a percentage. This often fools people into thinking they have to enter their new size as a percentage. However, you can enter the size using pixels, centimeters, etc... by specifying the units when you enter the number.

Positioning

The top-left corner of the canvas is the origin. That means the point (x: 10px, y: 20px) would be 10px to the right and 20px down from the top-left corner of the canvas.

The blue arrows show where the new position of your object can be entered. By default, the center of the object will be placed at the position that is entered. The green arrow shows where you can change the default reference point; I usually use top-left.

Apply / Cancel

The yellow arrow point to the Apply (Ctrl+Enter) and Cancel (Esc) buttons. Use these when you are done.

To perform your specific task:

1. use the marquee tool to create a selection
2. click: Selection > Transform Selection
3. specify your desired settings and apply
4. fill the selected area with the color of your choice
5. repeat