First Gear has a detailed built-in help system. The following is an excert from the section on finding a bike that is the right size for the intended rider.
Body Measurements
By knowing the lengths of various parts of the riders body, First Gear can suggest a good starting point for the size of your next bike. Some measurements are more critical than others, and the quality of the subsequent results are reflected in the amount of time and care taken during measurement. Consider the following.
In a hurry and only need a ball-park figure?
At the very least measure your height, and do it accurately. If you only have time to take a single measurement this should be it.
Have time to spare and looking for a better fit?
Takes just a couple of minutes more. An assistant is necessary for truly accurate measurements.
From the built-in help...
Taking all these measurements can take quite some time however, so first Gear provides a very quick way to estimate any measurement you don't have the time or desire to measure. The estimations are based on real data gathered from thousands of adults worldwide and can be very close indeed! Additionally, some measurements are more important than others, so you may choose to measure the more important ones, and leave the estimation of the remaining ones to First Gear.
At the very least, you need to enter your height and gender. For your height, fill in either the metric field, or the feet and inches fields. It is strongly recommended that you also enter the standover height. If you have time, measure the remainder of the advanced parameters. If you still have time, measure all the non-critical parameters as well.
When you are done measuring, click on the estimate unknown values button. This will 'fill in the blanks' for any measurements you did not take.
the final step is to click the suggest ideal bicycle button and find out what bike suits you best. Of course, the more information you supply, the better the suggestion will be.
A no-brainer really. This is your height; bare feet please!
Choose your gender as well. The dimensions of men and women differ,
and if you tell First Gear your gender, any estimated lengths
will be more accurate.
This is the height of your groin, measured while in bare feet. A
popular way of measuring this is to stand with your back to a wall and
a book up as high between your legs as is (comfortably) possible. The
book provides a convenient horizontal surface to measure to; once you
have found the height where you can still keep your feet flat on the
ground, carefully hold the book in place. Now 'get off' the book, and
measure the distance from the ground to the upper edge of the book.
This is your standover height.
This distance is important because it tells you the maximum height of
the top bar of the bike that you could safely straddle.
The distance between the centre of the hip joint and the acromion (the
pointy bit at the top of the shoulder). It can be helpful to move
your leg around a bit in order to locate the exact spot of the hip
joint.
The distance from the acromion (the pointy bit at the top of the
shoulder), to the elbow.
The distance from the elbow to the wrist joint.
This distance has no significance for bike fitting. It is useful
however for simulating how the rider looks while on the bike.
Probably not worth measuring unless you have oodles of spare time.
The distance from the wrist joint to the tip of the middle finger.
Not such a critical distance, but useful for simulating the riding
position on the bike.
The distance from the centre of the hip joint to the knee joint. It's
probably easiest to measure this when standing. Long thighs and shins
mean you can comfortably spin longer cranks.
Height
Standover height
Upper body length
Upper arm length
Forearm length
Shoulder to ear distance
Hand length
Thigh length
