Go to Settings -> About. It'll tell you. I'm pretty sure it's 1280x768 but you might as well check for yourself
Edit: ok,i see what you mean. It doesn't actually matter; so long as it's set to 1280x768 it'll be the right size. I don't know why there is even a "ppi" setting; that depends only on what display you are using. FYI the lumia is something like 330 ppi.