How to make a diamond shape

There may be more than one way to accomplish your task, but consider to create two lines with either specified angle and with the appropriate lengths. Duplicate the two lines and rotate the copy, then join the ends using the node tool.

Make a square or rectangle with the sides you desire then place a vertical line near it. Select the line and rotate by 30 degrees. That will be your guide for skewing the rectangle with the skew tool. (the square that I highlighted in the screenshot)