There are 2 ways you can resize an image
Edit > Image Size > and adjust your height and width. You would want to set it to Percent over pixel where it scales properly.
The other method, which I use the most is the transform form method.
I set the canvas demensions that I want then go to Edit > Transform > Scale. THen I click on the chain icon to keep the aspect ratio equal and adjust from there. It can have its problems depending on the image size and how you want to scale but its a lot better than guessing.
