You can use the Rectangle tool in the Create Sketch toolbar menu to quickly draw a rectangular face. Extrude the face to create a rectangular volume.
Keyboard shortcut: R
Use the axes to ensure you're drawing in the correct direction:
You can draw rectangles on the X, Y, or Z axis. As you draw, you can specify a dimension by simply typing in a value, which will bring up the Edit Dimension dialog. Hit Enter or press OK to accept the dimension.
Line colors by axis:
X = Red
Y = Green
Z = Blue
When you finish drawing the rectangle, it becomes a face that can be extruded to create a volume.