First you want to expand the canvas size, by doing so click file, then new. Put the height and width px of the size you want the picture to be. THen go to files and open up the picture as a layer. After doing so, it should pop up, then you want to go to the tools and hover your mouse over the one that says re-scale. Ignore the little box and click on the picture and drag. When it's at the size you want click ok.