don't size the image in photoshop, you'll only stretch the image. if the image isn't at least 5500px, you won't get a 60" print out of it. if you adjust it in photoshop, you'll only mangle the image. and with a setting of 300dpi, at 60" when it started out as - i guess that would be like 12" at 300dpi - you'll increase the size exponentially and will have an overly stretched image that will have to be saved at a low quality. just ensuring it won't print. you'll just have to be satisfied that your images won't be printing at 60", but closer to around 48".
---Mike Savad