I've rode bikes all my life but only recently in the past year have I obsessed about them. Once one gets past all the superfluous stuff, it becomes abundantly clear that size and fit ought to be the paramount concerns when buying a new frame. I've learned the hard way over the past year as I have struggled to get a proper fit on my 62cm Mark V without breaking the bank (on stems, forks, seatposts, etc).

Now the purpose of this thread is to discuss frame-sizing. The pro's and con's of sizing up, sizing down, of square sizing, of longer top-tubes, longer-seatposts, of heat-tube and seat-tube angles, etc.

What I want to get out of this thread is a better understanding of critical points for intervention when fitting a frame and the process that has let many of you to finding the perfect fit. I know alot of it is trial-and-error, but there are very concrete strategies for dialing down a bike so that riding is a pleasure rather than frustration.

To start, my first question is about square frame sizing versus not. What are the benefits of a square-sized frame, if any, and why isn't it popular to build frames in this manner?