From what you describe, it doesn't sound like any problem for a kingpost bent or maybe even a queen post. You wouldn't need any center post. The tie beam will likely be between 8X12 and 8X16. The larger depth is often needed to accomodate the floor joist mortises. You then have a number of options for floor joists. You can have 12 foot joists all the way across or split it up with larger beams and then smaller shorter joists. It depends on what materials you have to work with or what look you want. If you are not supporting a roof with this loft, well that is even easer with less load. You'll stil want beams in proper proportion though so they look right. Have you learned joint stress analysis? A good start to understanding joint stress analysis and sizing of beams is Steve Chappell's book "A timberframers workshop". Always have your worked checked by someone qualified. I'm an engineer too, but I always want another good set of eyes to catch my mistakes.