Forgive me if this has been answered or is covered somewhere else but what is the Pixels per inch the site uses? (Or does it change from one monitor to another?)

Do you mean the screen width of DD? Because that will vary depending on what part of the site you're looking at and the content inside the format at the time, but normally I think the standard is 1024 pixels wide.

As for actual DPi that depends entirely on you own monitor. Most monitors are either 72dpi or 91dpi (for Macs) I think...