When I scan (and I use Paint to scan), I add to the image size by clicking and dragging the corners of the scanned image. I usually have to use the zoom feature (zooming out) to make the image 25% of its true size so I have room to click and drag.

This takes an image the size of, say, a business card, and adds white space to make it the size of a book. I then click and drag to center the original image, and save. Now I have my original image surrounded by more white space in the margins.

As you noted, when you straighten in Picassa, the image grows. When I use Picassa to straighten the image, I have all that extra margin/white space that gets absorbed as the image grows. When I am finished straightening and have saved, I then crop to get rid of any excess margin.

This makes it a 3- or 4-step process, but I have tolerated the inconvenience. I like the fact that Picassa saves your image with the original dimensions & file size, so if I crop a photo it does not reduce the file size. That way I know it won't become too small to meet eBay's minimum standards for photos. (if you take a 1 MB image in Paint and crop it to half size, it becomes 500 KB or something similar; in Picasssa, it stays at 1 MB)