The command displays a dialog which shows the dimensions of the original layer in pixels. You can set the new Width and Height for the layer in the two text boxes. If the adjacent chain icon is unbroken, the width and height are automatically adjusted to hold their ratio constant. If you break the chain by clicking on it, you can set them separately, but this will result in distorting the layer.

However, you do not have to set the dimensions in pixels. You can choose different units from the drop-down menu. If you choose percent as units, you can set the layer size relative to its original size. You can also use physical units, like inches or millimeters. However if you do that, you should pay attention to the X/Y resolution of the image.

If you enlarge a layer, the missing pixels are calculated by interpolation, but no new details are added. The more the layer is enlarged, and the more times it is enlarged, the more blurred it becomes. The exact result of the enlargement depends upon the interpolation method you choose. After scaling, you can improve the result by using the Sharpen filter, but it is much better for you to use a high resolution when scanning, taking digital photographs or producing digital images by other means. It is an inherent characteristic of raster images that they do not scale up well.