do you mean you made the picture larger?
If the latter, this is done by making the whole window bigger. The picture expands to fill the available space.
I was working on a JPEG in Preview, and somehow was able to stretch the image with my mouse vertically and horizontally within the box. I don't know how, and can't seem to do it today. Anyone know what I am talking about? I am using an iMac 10.5.8 and Preview 4.2.