Actually, there is no default export size when exporting a diagram to PNG. Either the diagram's current size is used (i.e. option "Size" is set to "Original Size") or custom dimensions are used (i.e. option "Size" is set to either "Custom Width" or "Custom Height"). In the latter case, the size of the generated image depends on the corresponding value from options "Custom Width" and "Custom Height". Note, only one of these values is used, the other dimension of the generated image is calculated such that the image has the same aspect ratio as the diagram.