Use Custom Image Size
This controls how 2D exports determine the size of exported images. If this option is false, exported images have the same dimensions as the view (the default behavior). If this option is true, the dimensions come from the custom image width and height instead. If you want to export 2D images of a particular size, set this option true and use the width and height to specify the size in pixels. For optimal quality, export your images at the intended size rather than resizing them afterwards with an image editor.