I'm sure everyone else already already knows this, but I have just noticed it this week, so I post it here for other newbies like me.
In any orthogonal view (top, left, etc), when you draw a rectangle in empty space, the "Measurements" box shows the horizontal dimension first. And that's how you use the numeric input in this case: horizontal, vertical.
If you draw it on a face, however, the measurements box shows the longest dimension first, no matter if it is the horizontal or the vertical. Thus, the order in which you should input the dimensions depends on how you start to draw it. So, if you want to keep your sanity, you might want to start drawing it with a longer horizontal dimension before using numeric input.
Hope it helps