New pre hung interior door is 28" and the old area was 29 11/26 is this too big of an area to use this door?
Is this a split jamb door with the casing already attached?  If so, then if the casing hides the gap with room left on the outer edge of the casing to fasten, the door will work fine for the rough opening.  The most important part of the installation (other than plumb &amp; level) is that you cut or otherwise use snug-fitting shims frequently -at least, one shim behind every hinge, one shim near the door strike with one or two more above and below the door strike.  You can't overdo it, good shimming is what keeps the door in place through the years.  Important: make sure to put a nail, or better yet a trim screw, through the jamb, through the shim, and solidly into the framing.  After that, nail the casing, fill, caulk and paint, and you're all set.