It's not in the render options. Simply click on the "render" button just next to the "previsualisation" one on the main window. Here, click on the size, a new window will appear where you can change the desired size and resolution. 
For the PNG save, it's filesave as after your render finished. Or click as to close your render, you will be asked if you want to save or not, here you can specify the format.

Hope it helps.

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