I have just been debating with myself the best way to get a 13x19 image out of my 20D and still have it look decent.
I was printing 12x18 from 20D after enlarging in Photoshop keeping 300dpi and changing the image size measured in inches.
They look great when viewed at a proper distance. After all, you don't usua;;y look at prints this size form 6 inches away

I was surprised that they survived a single step enlargement like that, when I do single step reduction I get jagged lines and it works much better in several 75% resizes.