You didn’t post a link or explicit reference to the 3DW model you are using, so I can’t make any specific comments about it. A quick search returned 4138 models involving “shed”! It would make life much easier for everyone if you would upload the model here so we know exactly what you are dealing with.
To get an exact size using the scale tool, first click the desired handle, move the mouse, and click. Then type the desired size into the VCB (aka “Measurements” or “Scale”). Be sure to include the units or it will be taken as a ratio instead of a size. You should be aware that scale will resize all parts of the selection proportionally. For example, if you scale vertically the thickness of a horizontal board will also scale. That might not be what you wanted. Often it is better to select the geometry of the end and reposition it using the move tool. The edges attached to it will stretch or shrink as required.
The arrange menu is in Layout, not SketchUp.
You can always edit a model to add layers, create components, etc. I can’t tell you why the author of that model decided to build it the way he did. The 3DW has no quality control, and submitters are free to post whatever kind of model they like.
Again, without knowing what model you are talking about I can’t tell you why you can’t take the siding off. Maybe it is in a Group or Component that you need to open for edit first? However, this has nothing to do with layers. In SketchUp, layers control visibility. They do not encapsulate, isolate, or otherwise organize anything. Misunderstanding SketchUp’s layers is a common issue for people who previously used other CAD or Photoshop, where they mean something entirely different from in SketchUp.
The difference between Groups and Components is subtle but pretty fundamental. Groups are meant for one-of-a-kind collections of geometry. Components are meant for collections of which you will want multiple identical copies. If you edit a Group, you affect only that one Group even if you have copied it. If you edit a ComponentInstance, you simultaneously and identically affect all the other instances.