Find the YourApplicationName-app.xml file in the src folder of your project.

You will find following lines in it

<!-- Whether the user can minimize the window. Optional. Default true. -->      <minimizable>true</minimizable>

<!-- Whether the user can maximize the window. Optional. Default true. -->
<maximizable>false</maximizable>

<!-- Whether the user can resize the window. Optional. Default true. -->
<resizable>false</resizable>

They will  be commented.. uncomment them and change the values to true or false as per the requirements.