If you need bigger files, have a look at this upsampling technique on
Digital Outback. I've used it with good results to make a 90 cm wide image from a file only 3100 pixels wide - if I had made a straight print that would have only been ~85ppi. It also useful for upsampling to send images to labs that demand 300ppi images.
There is even a photoshop plugin to do the work for you
