What's the "proper" way of deciding what size bike

Okay, bear with me because I know it's a stupid question but just how do you decide what size bike is "proper" for your build?
What has me asking is I've been ringing around about getting a retro style road bike built, not really seriously as I'm skint till I find someone who will employ a student but just to see what's available at what price and more than one builder has said "come in for a fitting and we'll take some measurements.
I ride a 57cm top tube pug, I wasn't aware I'd need a fitting I hadn't even considered that the bike could be the wrong size as it's comfortable for my 5 10 build, so how do you know? What measurements would they be taking? I'd been asked my inside leg before and my "reach" would there be more measurements than that? How would this translate in a custom bike?

If you are looking to get a frame custom built, rather than buying a used or NOS retro frame, then a good frame builder will most likely want all your measurements or a visit to sit on a rig where your best seat height and reach can be worked out.
Try searching for the BBC documentary 'It's all about the bike' by Robert Penn. That shows him being custom fitted at a well know frame builders.
Without obviously knowing your size other than 5'10" I would say that a custom built frame would be smaller than your 57cm pug.

It's not a stupid question at all. Frame length is really important too. Essentially if you get yourself measured up on a bikefit jig, then you can work through with a frame diagram and work out which size fits best.

Okay, so say my measurements are x how do they then take that to produce tube length y? Is it something that would be intuitive or is their a science or mathematical ratio to it?

How come you reckon it would be smaller? Would it be tighter clearances or something more specific?

I've seen that video but it didn't really give any clues on how they work it out, the place he went to, Rourkes, is actually my home turf being a Stokie in Leeds.

a smaller frame is just my experience, I'm taller than 5'10" and although have 3 frames of 57cm from my racing days in the 80's & 90's I find a 54cm fits me more comfortably.
An experienced fame builder will have a set of usual top tube lengths depending on your reach. I seem to remeber my mate having a raleigh frame built from a set of his measurements, rather than him needing to visit the work shop.
Gonna be an expensive job though, which is why every man and his dog chases 531, 653, 753 or columbus frames on ebay.

The top tube length will depend on the angles of the frame and the wheelbase as well as the seat tube length. It will also depend on the builder's interpretation of what you want from the bike and their own normal practice.

I have two 80s retro frames which theoretically fit me but they are quite different.
My MKM was designed as a short wheelbase frame in the British time trial tradition. It has a 56cm c-c seat tube but only a 53.5 c-c top tube. It also has 74 degree parallel angles. My Daccordi on the other hand has classic Italian geometry as used by lots of different builders in the 70s/80s. It has a 54cm c-c seat tube with a 54.5cm c-c top tube. The angles are 74 seat and 73 head.

Obviously I use different length stems to compensate and brake lever positions also play a part in the fit.

Mark.

Hmm, okay that all makes sense!
I suppose to learn the frame builders art you have to be a frame builder!
I'd never twigged that stems could be different lengths either! So much to learn!
I was going to ask about geometries but I think that's pushing it a bit!

If we're talking about off the pegs and new old stock would they be subject to the same considerations?

I had a Raliegh factory custom written off early in the year getting car doored, I was gutted needless to say.

you might find this helpful to find out the frame size that best suits:

http://www.competitivecyclist.com/za/CC ... ATOR_INTRO

Great find, I'll check it out now!
Then it's back to bikecad.

Often forgotten is head tube length. This will affect how upright you want to be - if you want the Italian arse in the air pro look a short headtube will achieve it. A more comfortable and slightly more upright stance will need a longer headtube which can be aesthetically more pleasing than an over- stacked up steerer tube or a flipped stem.