You can resize the SourceTree main window by clicking and dragging the left, bottom, right and both bottom corners of the window...  You cannot resize the window by the top edge or top corners

This is one of many many issues introduced in 2.x, Please do better testing before releasing new versions, it seems with each update that 1 bug is fixed and 5 more are added. For the last few years I have been using source tree it has been going consistently downhill.