When you do a Google search for information about choosing a frame size and fitting a bike you'll find a huge number of articles. How do you choose whose advice to follow? Also, at the end of the day the choice of frame size for all but the top end frames is going to be a simple multi-choice with most frames coming in 4 or 5 set sizes.
The starting point for selecting frame size is usually matching your height and inside leg measurement to the stated size of the frame. For most bikes this is the measurement between the centre of the bottom bracket to the top of the seat tube. The online bike shop Wiggle have quite a good page on bike fitting that covers the basics but also introduces perhaps the most important concept which is that the type of bike and what you intend to use it for makes a big difference. Following the general advice on that site I'd expect to be looking at a 48-50cm road frame or XS-S compact frame. I particularly liked their 'Ape Index' which introduces the idea that reach is also important and can be used as a sort of tie-breaker when you seem to be on the boundary between sizes. Unfortunately it doesn't help me as my arm reach is basically identical to my height putting me on the boundary there too.
If you want to dig a bit deeper then you'll find that Wikipedia is a great resource for cycling information. The page on Bicycle Frame Size goes into all the details.
Another highly respected source in the world of cycling is the late Sheldon Brown. Sheldon Brown's website is a fantastic treasure trove of articles and advice on all aspects of choosing and maintaining bicycles. His articles tend to go into a little more depth than other sites but I like his writing style and the way he is prepared to challenge existing ideas. Unsurprisingly, in his "Revisionist Theory of Bicycle Sizing" he comes to the conclusion that the seat tube measurement should be secondary to the top tube measurement (the tube between the saddle and the head tube).
To help me visualise this problem I drew some pictures. Using the published frame measurements it is possible to draw the range of saddle and bar positions for different frames, relative to the bottom bracket. (The position of the bottom bracket relative to the ground and the wheels will change slightly with frame design and size.) I used the adjustment ranges on my Cinelli Vai Bianca seat post and Selle Italia C2 saddle.
My gut feel is that too much seat post inserted in the frame is as bad as too little, more contact area to get stuck perhaps? In fact, the Saetta came with a 130mm alloy insert for the seat tube which you can see in the following picture. Note that the fixings for the bottle cage lugs and 'braze-on' adaptor can be clearly seen in the tube (and will prevent the seat tube from being inserted too far anyway).
Moving to the front of the bike, a similar picture can be drawn for the bar position. In this case the shaded areas result from changes in the number of spacers on the steering tube and the length of the stem. Again, I chose to compare using a single product for comparison (the Cinelli Vai Bianca stem). You can get shorter or longer stems but the extremes seem best avoided if possible (as they leave less wiggle room later). However, there is also considerable variation in stem angle which can be used to raise and lower the bars. As far as I can see carbon frames tend to have taller head tubes (or it may just be the fashion of the day) but combined with the shallower angle of the Cinelli stem it makes for a higher bar position on the selected frames (key as above).
If you are having difficulty picturing the way these diagrams fit with the bike, here is both diagrams with the (fixed) bottom bracket position shown and the wheels added for context.
As I drew these diagrams (with a little help from my computer!) I did get to thinking about the triangle made by the three contact points: the cranks, centred on the bottom bracket, the saddle and the bar. It is possible to rotate the triangle while keeping the same shape. Rotating backwards will take a little of the weight off your arms while likely increasing your wind profile and rotating forward will do the opposite. This point is made in another article on Sheldon Brown's website: The Myth of KOPS. As a result, I marked rotations of 1 and 2 degrees either side of my current position on the diagrams which explains the smaller grey circles.
As you can see, within reason, adjustments to the stem and the saddle/seat-post can be used to adapt different frames to the same rider. There was certainly a time when people were advised to ride the smallest bike they were able to get away with. The argument then was that smaller frames weighed less. These days, with materials like carbon fibre being affordable this may no longer be true. Heavier riders can probably usefully use the extra strength provided by the larger frame sizes. But as for the variation between manufacturers, perhaps the initial conclusion in "Bike Frame Sizes, Geometry, Angles and All That!" is valid in this price range: you might as well choose on the basis of colour.
So how did I choose? Choosing a frame that I could source through my favourite bike shop was high on my list of priorities along with a little brand loyalty and romance (read 'colour') for Columbus/Cinelli. The handling of my current bike could be improved by moving my centre of gravity back a little and increasing the distance between knee and handlebar - yes these have been known to collide when I'm out of the saddle at the end of a long day! Strangely, the Saetta M is clearly too large for me but the S and XS are very similar in size. I went for the S, rather than the XS - figuring that the slightly longer wheelbase might be more useful for reducing toe-overlap.