Scaling to an exact target size is tricky... and should never enlarge!
Andrew from United States, using photoshop cs with mac os x, writes in with:
[Quote:] How can I scale (ie image size) an image based off a slection? I can't crop the image because I'd like to have a specific border treatment based off the area outside of the selection. For example, I have a face that I'd like to be 400x400. I make a square selection around the head (which happens to be 200x223). How can I scale the image based of that selection?
1. Use the marquee or lasso tool to make your selection.
2. Goto Layer > New > Layer via Cut to put your selection into its own layer.
3. Hide the background and all the other layers.
4. Use the resize wizard (Help > Resize Image) to enlarge the selection to the desired size.
5. Use the Marquee tool to select the image from the wizard window, and Edit Copy (or Cut) to load it into you computer's clipboard.
6. Close the wizard image window (save it as a file for safety if you want).
7. Back in your original file, hide or delete the layer from which you made
8. "Unhide" the background and all the other layers.
9. Create a new layer (Layer > New > Layer)
10. Paste (Edit > Paste) the image from the wizard into this new layer, and use the Move tool to get it into the right position. If your calculations are correct, it should fit.
Be careful. You're scaling a 200 x 223 selection into a 400 x 400 hole.
200 x 223 is NOT square and will distort the image.
You are also scaling the image UP which is going to introduce some image degradation -- not recommended to scale UP.