I would say that, in my opinion, 200-250% might be pushing it a bit. If you do want a large print from a smaller file you could resize it in Photoshop to get you part of the way and then let our Lightjet 5000 printer interpolate up from there. In our experience the Lightjet’s interpolation seems to do a better quality job than resizing in Photoshop does, but using a combination of the two methods might get you to a bigger size print than you might imagine possible from a smaller file. The Lightjet can interpolate up from half the dimension size (which is a quarter of the file size) without any noticable loss in quality. So for a 20″ x 16″ print you can supply us with a 10″ x 8″ at 305dpi. The added advantage of supplying files at half size is that upload times are considerably reduced.
Click on the following link for further information in an earlier blog post about Lightjet resizing and interpolation