Bike Friday has 8 sizes from 48cm to 62cm with 2cm difference between the sizes, the explanation they give is:
A Bike Friday frame is sized according to 8 “effective” top tube lengths. The “Effective Top Tube” is the distance from the center of the headtube to the center of the seat tube measured horizontally. This number is equivalent to the top tube length on a standard pre-2000 road bike with a horizontal top tube.
See
https://bikefriday.com/faqs/ question "
What FRAME sizes do you make, and how do they compare to a standard or non-folding bike?".
When you order a Bike Friday, they ask you to measure and provide the values of a bike that fits your needs.
I measured my Birdy frame with the saddle and stem set for me and provided the values to BF and this ended up with a BF frame size 62, i.e. the biggest BF frame size available and indeed, this BF 62 frame size match the sizing of my Birdy while the frame size of my made to measure race road bike is about 55cm (I am 180cm, which requires usually a frame size 54 or 56 for a race road bike).
So there is a big mismatch between their size explanations (according to which I should have asked for a frame size 54 or 56) and the size they computed based on my Birdy. The 62 frame size I received provides a similar position on the bike as my Birdy and fits perfectly my needs, a frame size 54 or even 56 would have been much too small.