how to set the size and is it possible in the free version?
Normally you would start by drawing a rectangle entering the desired dimensions. Then use Push/Pull to extrude the required distance. With the box you show, you can resize it by using Push/Pull on the faces or you can select a face nd use the Move tool, or you could use the Scale tool. This is the same in both SketchUp Pro as well as the SketchUp for Web versions (including SketchUp Free).
Sounds to me as if you should take some time to go through the instructional content at learn.sketchup.com as well as the Square One videos on the SketchUp YouTube channel.