Need Help Resizing/Editing Photo

[Mr.Maduro]

I created a photo collage on my computer that I want to bring to a photo place to print, but at the photo place, the photo is partially chopped off on the display and I'm getting a warning that the resolution is too low.

Is there someone who can help me resize the collage so that I can print it high resolution and a particular size? Whoever helps me out will get a tasty fiver for their time/trouble.....