Most popular frame size?

In the process of having a go at building my own frames and just for future reference, what is the most popular frame size?

I know it's a 'how long is a piece of string' kind of question, but I've always found it hard to sell real large frames and small frame have always been easy.

That's because more people are smaller than average than are larger.

Let's be clear - you're talking about horizontal top tube/traditional geometry frames?

I'm not sure how or where you are selling your 'real large' frames but as a taller rider i can assure you that good frames over 60cm don't hang around for that long.

57-58 zooms into an early lead

I've always found it hard to sell real large frames and small frame have always been easy.

Could be because it's easier to set up a too-small frame with longer seatpost/stem etc. And long seatposts look coooool man.

Also, I bet most people are riding frames that aren't the correct size (me included).

My initla thought was 56cm

Lets face it, frame size is a variable commodity,
Traditionally, on a conventional horizontal top tube frame, the size is the length of the seat tube, centre to top (British) or centre to centre (Italian) and there's about 2 cm difference in the two measurements.
With modern compact geometry, actual seat tube length is irrelevant so a virtual seat length is sometimes quoted, but (virtual) top tube length is more important.
Many manufacturers sell frames in 2cm increments, 50-52-54 etc, but these numbers bear little resemblance to the length of any actual tubes on the frame.
And others just go for sizing in the small-medium-large way.

So what I am saying is that 2 frames in the same nominal size may actually be completely different in reality.

This will be why i'm struggling to find what i'm looking for in 56cm then...

Which is why, when you are buying a new frame you should look at the full spec and not just the quoted size.

What the...

This graph amost represents the distribution of human scale in caucasians?

(or is it an island populus)

You should, I suggest, provide a model for measuring frame size. There are a number of measures in use.

• center of bb to center of top-tube (c-c)
  
• center of bb to top of top-tube
  
• center of bb to end of seat-tube
  
• top of bb to center of top-tube
  and, for sloping top-tube frames, the measures with a virtual horizontal top tube. A number of vendors don't even bother but instead list their framesizes like T-Shirts as S,M,L with the determinant factor being the effective reach.
  
  

Two identifical frames from the same OEM can have under different brands quite different listed frame sizes.

Given the average height in the UK is 5'9 1/2" (and just over 5'10" in the US) I would expect average frame size of 54 cm c-c for advanced recreational road cyclists in the UK. While cyclists are not uniformly distributed among the population I would still expect 53-55cm to be a good average in the US and UK given the dominant fitting models.