Image size printing?

I'm making my prints at Mpix.com, But I want to make sure my pictures do not come out pixelated or with bad quality, I use the nikon D5100. If I'm making big or small prints what is the image sizing, resolution and quality # that i have to put in lightroom? Please help I don't understand for me it a bit confusing.

Image quality will be set by the camera, i.e. If you have a 10Mp camera, then the image quality is 10Mp. In Lightroom, you can change pixels per inch, but if you want quality output, then do not go below 300dpi (which is default I believe), and don't try to go denser than 600dpi or you will invoke interpolation, which will cause your quality to suffer.
Scaling is done programatically (by lightroom), and as long as you aren't trying to blow a postage stamp into a poster, your output quality should be as fine as the printer can get it. There are many determining factors… Printer quality is one, some printers can print super high quality (i.e.capable of actually printing 600dpi… For comparison, ordinary magazine slicks are 300dpi)
Scaling by the program, making it smaller is fine, bigger can be dicey… Check the size of your native (original) image. Mine come in to Photoshop huge, so scaling is seldom a problem.