In WM 4 (the LTE versions), you have the option to (slightly) enlarge that preview window in the project bar. You can, in WM 4, also split up your screen in 1-4 panels, and for example have a Device Overview and 3D Overview at the same time. For both, see the image below.
In WM 3028 and earlier, you are stuck with the only option, that is not available in all versions, namely, opening up a separate window. You do so by clicking on this option:
As @HYLK says, you can resize in the 4000 series builds. There is a plan to allow you to float the preview window (as well as the other sidebar components) separately, although I don’t have an ETA on that right now.