Just to confuse you further: it won't look pixellated, as such, until it's very big relative to its original size, but it will look coarser. When you make it bigger, Photoshop will resample it with a reasonable filter rather than merely duplicating pixels to make it to the size you've asked for.

This will tell you many things you didn't think to ask about (http://www.amazon.com/Real-World-Scanning-Halftones-3rd). One is that dots per inch is a terrible measure unless you know exactly what your printer is doing, and even then it's suspect. ;)