A rectangle's dimensions dynamically appear in the Measurements Toolbar as you draw. Specify exact length and width dimensions by typing them in the Measurements Toolbar, and pressing Enter (Microsoft Windows) or Return (Mac OS X) either after the first corner is clicked or immediately after the rectangle is drawn.
SketchUp will use the current document units setting if you only type in a numerical value. You can also specify either Imperial (such as 1'6") or Metric (such as 3.652m) units at any time, regardless the document units setting.
You can also type one dimension at a time in the Measurements Toolbar. If you enter a value and a comma (3',), the new value will be applied to the first dimension, and the second dimension will be retained from before. Similarly, if you type a comma and then a value (,3'), only the second dimension will be changed.