To be honest, for displaying purposes like in here, a 600px size would be ok. The loading time would be faster and the average screen quality would handle properly. Now, for the exhibition I don't know because it depends what they are going to do with the pictures, if they are going to print or not, and the size they are going to print them as well. But if they are for Mobile pictures, then they will stay on the average 8 megapixels output.
Upscaling too much is not always good, it depends on the kind of picture. Bigphoto is good but if you don't upscale too much, even if they say they can handle until 600%, I wouldn't go more then 200%. For professional purposes there are other desktop softwares that do a great job with that but they are quite more expensive.
It really depends from what they ask and from the quality and size of your pictures.