If i have the default cube and then go into edit mode and add another cube, I cannot find where to set absolute dimensions of the second cube. I can scale it, but is there a place to find where you can set absolute dimensions? The original cube and the second cube are treated as one if you go back into object mode.
Keep in mind that when you add a primitive (such as a cube), you can only adjust its defining dimensions immediately after creating it.
Once you add the second cube you can find the controls to adjust its dimensions at the bottom of the left hand toolbar (accessed by pressing T), or by pressing F6. As soon as you perform another action Blender "forgets" that the added mesh was a cube and doesn't consider its geometry any different than the rest of the mesh so from then on you can only adjust it's size by manipulating the geometry directly.
As far as I know, setting the size of a model joined with another model is not possible without selecting all the verts of the model you want to change manually. What you can do instead, is select all verts of one of your meshes, separate it into another object, and parent the two objects, depending on what you'll use it for. This links basic properties such as location, rotation and scale to both objects. If you scale the parent object, your child object will be scaled accordingly.
To separate verts to two objects, select all the verts you want to separate into another object, and press P, all in Edit Mode of course. This will "cut out" the selected verts and place them as another independent object.
Hope this helps.