Aperture 1.5.2 includes a new Scale watermark setting in the export setting window that addresses issues regarding how watermarks are scaled when applied to exported images.
If the Scale watermark box is left unchecked, then the watermark will appear on your exported images at its actual pixel size, regardless of how your images are scaled. This replicates the behavior of watermarks in Aperture versions prior to 1.5.1. Let's say for example that you create a watermark that is 100 pixels by 100 pixels. If you apply the watermark to an export that is 50% of the Original Size, with the Scale watermark setting unchecked, the watermark will be 100 pixels square. If you export the same image at Original Size, the watermark will still be 100 pixels square (and will, therefore, take up less space on the resulting original-size image). Below are some examples:
Here is an image exported at it's original size without Scale watermark.
Here is the same image and watermark exported at 50 percent of original size, again without Scale watermark.
In this workflow, you would typically create watermarks of various sizes to use, depending on the final export size of the images.
If the Scale watermark box is checked, then the watermark size is scaled relative to the export size. This replicates the behavior of watermarks in Aperture 1.5.1. In this case, the size of the watermark (in terms of actual pixel dimensions) changes, depending on how the image itself is to be scaled. In this workflow, the watermark is automatically reduced in size so that it proportionally always takes up the same amount on space on your resulting exported images. Here are some examples of images exported with watermark scaling:
Here is an image exported at its original size, with Scale watermark enabled.
Here's the same image and watermark exported at 50 percent size, again with Scale watermark enabled.
Note however, that watermarks are never scaled up in size. If the export size is larger than the original image, the watermark will not scale.