Lets try and cover all the bases :wink:
OK Spud, you'll notice that the top table is good and long and has the blades in the centre of it. As the others have said, this is for planing any size workpiece that can be slid along the table. Now you can slide your piece of 7" along that no problem, then do the other side and you'll have two lovely flat, shiny sides.
However, they may or may not be 7" apart all the way along, i.e. the thickness may vary, say 7" at one end, 6.5" at the other. There's no way of rectifying that on the top bed.
Now this is where you'd normally move onto the thickneser section. Once you've got one smooth, flat face after using the surface planer (top tables), you'd pass the piece through the thicknesser with that flat face downwards. That way it runs across the shorter thicknessing table while the blades trim the top face to thickness the piece.
Obviously as you've found, you can't thickness a piece thicker than the max thickness of your machine.
Benefits of using the thicknesser include often a better finish than the surface planer section, because the workpiece can't vibrate anywhere near as much while being power fed through, against a cast iron (or other) thicknessing table. YMMV.
The P/T is very much your friend, especially once you've got the hang of it and the blades are sharp. The drill for right angled stock goes as follows:
1. Surface plane one wide face
2. Surface plane one narrow face, with the plane wide face against the fence
3. Thickness plane the other wide face
4. Thickness plane the other narrow face (order of last two not vital)
Have fun
Martyn