Ah, I see. I clearly thought you meant you didn't read it all if it got too long content wise, not width wise.
But still, I feel wasting space isn't the right answer. Maybe percentage fonts, like frost mentioned, but on my screen that would look kinda dumb. Perhaps let the side bar width and the post width grow with wider screens together, and have fonts get a smallish percent size? Realize on lower resolution monitors, things look bigger.
I agree that posts feel a bit wide, but I'm used to reading books that are large in size and small in font, so it doesn't bother me at all. It could be a slightly smaller width, but make sure that the sidebar is better used in that case.