The image you see depends on two things: the preview scale and the zoom factor, both of which are visible in the status section of the middle toolbar, on the left side of the PostworkShop window and underneath the image.
When you first load an image, PostworkShop calculates the preview scale to fit your image into the window. Let's say that with your particular screen resolution the preview window is 1000 pixels. If you load an image that is 2000 pixels, the preview scale will be set to 0.5 and the zoom factor to 1.0 (100%). This will fill the preview window with the image. If you load an image of 500 pixels, the preview scale and the zoom will both be set to 1.0 and your image will not fill the preview window completely.
All calculations are executed on the preview image, so what you see is not the result of a full image render that has been scaled down. It is calculated on the preview image. A final render, that uses the camera icon, or a “Save” command, will force a full render. In that case, what you see is a scaled down version of the full render, but you can zoom into it, even during the render, with the mouse wheel. If you zoom above 100%, the displayed image will be the result of an interpolation, and is no longer a representation of your final image.
When using the zoom feature, the ctrl + double click scales the preview image to 100%. So if your original image is 2000 pixels and your preview scale is 0.5, the 100% is a 1000 pixel image.
Interpolation depends on the preview scale. If your image is 2000 pixels and your preview scale is 0.5, zooming into an image on a 1000 pixel preview window will be just an interpolation. But if you go into Options/Preferences and set the preview scale to 1:1 (1.0), your 1000 pixel preview window will hold only a 50% zoomed image. If you start to zoom in now, you will discover new details until you reach 100% zoom.
Why don't we always set the preview scale to 1:1 and play only with the zoom factor as your favorite image editing program may do? Because a PostworkShop style can be extremely complex and an image can be very large. Throwing a very complex style on a large resolution image (e.g. taken with a 12 megapixel camera) the very first time you use the software can take a long time for you to see anything happening and you might not understand why it was taking so long. If you know what you are doing, and you know the complexity of your styles and the performance of your machine, you can always set the preview scale to 1:1 manually and work the same way as your favorite image editing program allows you to do.
Once you click the 1:1 button in the middle tooblar, the image remains set at a preview scale of 1:1. Even if you close PWS and restart it, the new image you load will be shown at a preview scale of 1:1.
See also 7.2 Resize the Current Object for more info and help about how to resize the image.