Originally Posted by DaveSSS

It is not necessary to work with a fitter to get a good position on the bike. A fitter can only look at the rider and take measurements. It's the rider who knows what the changes feel like and any rider with some experience can make adjustments himself and evaluate the results. Eddy Merckx was constantly tweaking his position - by himself, not on the advice of a fitter.

Frame size, by itself has no effect on the seatpost setback. That's entirely a function of the seat tube angle. The next size smaller frame might have a steeper STA or it might not. It depends on the brand and which size you're dropping down from. Cervelos all have the same 73 degree STA, so a smaller frame would not need more setback on the post.

You can even find examples where the reach is nearly identical on two frames sizes because the manufacturer increased the STA by one degree on the smaller size, while reducing the TT length by 1cm and the reach ends up the same, once the saddle is set to the same position, relative to the BB. The only thing that changes is the length of the head tube being 2cm shorter, so the bars can be postioned lower.