I'm working on both the second edition of Veni Vadi Vici,  and on the new book The Theory and Practice of Historical European Martial Arts, both due out this year. TTPHEMA (catchy title, huh?) will include the answers to just about every HEMA-specific problem that exists, so it will of course cover how to choose a sword. Philippo Vadi's instructions for the size of your sword are the earliest example of sword specs in fencing literature, so they are included in the relevant chapter in TTPHEMA. One of my excellent beta readers suggested that this section could use some clarification:

Note, Vadi does not give any dimensions, only lengths relative to your stature. As in geometry, the exact dimensions are not interesting, as they give just one specific instance of the principle. The proportions are everything, as they can be scaled in any direction without the form being lost. Using Vadi’s stated proportions, my sword should be 133cm (my floor-armpit measurement) and the handle about 21cm (my “span” is 21cm, and I have relatively small hands, so this should be a bit bigger). Allowing about 5cm in addition for the pommel, that gives us 26cm, which is also the length of the crossguard (handle plus pommel). With reference to Peter Johnsson’s article in the Park Lane Arms Fair Catalogue (March 2012), let’s have a brief look at the geometry of this weapon. Using the 26cm hilt length as the diameter of a circle, 5 such adjacent circles will give us an overall length of 130cm: quite close to the 133 of my floor-armpit measurement. So, let us divide 133 by 5 to get a diameter of 26.6cm for our circle. Adding in the connecting circles to create the vesica pattern that Johnsson has established gives us nine interlocking circles. Nine is an excellent number for this kind of thing, being a trinity of trinities.
The crossguard is placed on the circumference of the first circle, and is of the same width as the diameter measurement. Its thickness should be within the third circle, not the first, to avoid taking length off the handle. It is probably square in cross section, and the tips should be pointed. I see this as a shallow pyramid at each end, rather than a chisel point or a serious spike, but that is from a general impression of the illustrations, not hard data.

It's my policy to shut up and listen when my keener readers tell me something isn't clear, so I thought I'd provide an illustrated run-through of the geometry. For the book, I'll get somebody who knows how to do these things create a set of nice clean digital geometry drawings, but I'll need to tell that person what to draw. And then I thought you might like to see it too…

Step One:

Draw a straight line:

Draw a straight line.

Step Two:

Draw five identical circles that just touch each other, with a pair of compasses. The centre of each circle is on the line. I drew the first circle too big (5 wouldn't fit), so ignore the extra large circle at the left-hand end.

Draw five touching circles.

Step Three: 

Draw four more circles, the same size as the previous, with the point of your compasses on the point where the previous circles touch.

[You could do these nine circles in one step, just placing the point of the compasses on the point where the circle you have just drawn cuts the line]

Step Four:

Mark the line for the crossguard, by drawing a line perpendicular to the original line, at the point where the first circle cuts the line.

Step Five:

Draw the crossguard, to the right of the perpendicular line. The crossguard is as long as the diameter of the circle.:

Step Six:

Draw the rest of the sword:

blade:

The handle:

Note I drew the blade too wide the first time, so narrowed it here.

Step Seven

Colour it in and share it with your friends!

I made the blade green because that's the colour of Luke's lightsaber in The Return of the Jedi, my first Star Wars movie.

Some things should be apparent from this post.

1. I will do anything to please my readers. Within reason…
2. I really do not know how to draw. This is why I hire people…
3. I'm not much good at photography either.
4. I am a sword geek.
5. Editing is really hard, and I will procrastinate in all sorts of ways…

But, there you have it: applying Peter Johnsson's sword geometry theories to Vadi's sword proportions. You can see the man himself doing it in this rather cool video:

And if you don't already have Choosing a Sword, the second instalment of The Swordsman's Quick Guide, then you can get a copy free by signing up below: