It depends on what the model is and what dimensions you are changing. Very often (most of the time in my case) the Scale tool is not appropriate for changing dimensions. Instead the Move tool is the tool of choice. And in that case you just move by the distance you need. If the thing is 20 in. long and you want to make it 24 inches long, you move 4 inches. Much simpler math, too.
You can also enter final dimensions when using the Scale tool if it is the correct tool for the job. So you might be able to start scaling and type the desired dimension adding the units to get the size.
A note about components: you can open them for editing (right click on one and choose Edit Component or double click on it with the Select Tool) and then make changes to the geometry inside.