Whole page shifting left when I define a <p> width.

OK, I know I'm asking this question because I'm an absolute beginner when it comes to CSS but here goes.

Many of the pages on my website (older ones created with GoLive) Are just <p> <img> <p> <img> and on and on.

A page I am working on is all nice and centred and then I select a <p> and go "edit rule" and go to box and  change it from auto to 800px. I do this so the text doesn't go all the way across the page which makes it hard to read. When I hit apply it changes the box but the whole page goes from being centered to being left justified. Everything on the page is up against the left border. What on earth is going on?