When starting from scratch, the ideal way to find you correct size and positioning is through a qualified bike-fitter. Bike-Fiting today has become more of a science than the "Black Art" it once was. Most cycling communities today have access to a competent bike-fitting studio.
As a web based company, we do not offer these services. There are so many personal variables and preferences to be taken into account when determining your most efficient position that we felt an online "Bike Fit Bot" would give our customers a false sense of certainty.
If you have a previous bike that you are satisfied with your positioning on, however, emulating that position onto a new bike is actually quite simple.
We believe Stack and Reach are the only true references required to base your bike sizing on. These two values determine the position of the three contact points between your body and your bike (Crank position relative to handlebar as well as saddle position). Get your Stack and Reach right and you can easily match the fit of your old bike.
You should be able to find these numbers using a google search, websites like geometrygeeks.bike offer free access to a database of most of the common bike brand's geometry tables available online.
Saddle height and set-back can be adjusted after you recevie your bike. Except in the most extreme cases this is not a critical factor when selecting your frame size. For a great starting point for your saddle height, one of the most accurate and simplest methods was developed by Tour de France winner Greg Lemond in the 1980's; taking your leg inseam and multiplying it by 0.88 (ie 79cm inseam x 0.88 = 69.5cm seat height). From this starting point you can adjust a few millimeters up or down based on saddle, pedal type, shoe sole thickness and feeling, but its a real good start.
Proper "set-back" should position the tip of your saddle approximately 5-10% of your saddle height behind the vertical line drawn through the crank axle. You can measure this by dropping a plumb line off the tip of your saddle.
The 0.88 and 10% rules are good starting points and will put you close to a really good fit. You should be able to acheive this position on nearly any frame size, due to the large range of seat post extension possible.
Bar/Stem height is more limited by frame size and it is thererfor critical that you choose the correct frame. We recomend never using more than 20mm of spacers between the top cap of the headset and your stem, meaning you have only 20mm of adjustable height. Stem length is obviously more flexible as stem lenths range from 80 to 130mm.
We suggest you compare the Stack and Reach numbers for your current frame to the geometry of our frames. For exampe, if your current frame has a Stack of 550mm and you currently have 10mm of spacers under your stem, then by choosing a frame with 540mm of Stack and and using 20mm of spacers you would end up with exactly the same bar height. Likewise, if your current frame has 390mm of Reach and you use a 100mm stem then by using a 110mm stem on a frame with 380mm of Reach you would be within millimeters of your current position.