In inches, what was the hand-span of the violinist Niccolo Paganini?

Here's a partial answer. If someone knows the measurement in inches of an octave on the violin neck, they can calculate the hand span in inches.

"Another aspect of Paganini's violin techniques concerned his flexibility. He had exceptionally long fingers and was capable of playing three octaves across four strings in a hand span, a feat that is still considered impossible by today's standards. His seemingly unnatural ability might have been a result of Marfan syndrome or Ehlers-Danlos syndrome."

https://en.wikipedia.org/wiki/Niccol%C3%B2_Paganini

I found the figure "18 inches," from outspread thumb to pinky, on some classical music chat sites, but since I couldn't confirm it elsewhere, I decided not to post them. That figure seems extremely unlikely to me -- possibly merely legend -- but I don't play the violin. I'll leave it up to experienced musicians to decide whether it could possibly be true.

Regarding tfm's Wiki site, I noted that a reference was lacking for the claim of Marfan's or Ehlers-Danlos.

I've had to look about a bit for this and the 'Book of Lists' (no link sorry, just the book) says it is 18 inches - but that's massive! I've got quite a big thumb to pinky span (8 and a half inches), but you try measuring your hand span against 18 inches on a tape-measure, it's collosal! Has anyone got any definite proof/ links etc?

In an article from the AMA journal dated 2 January, 1978, Dr. Myron R. Shoenfeld advances the theory that Paganini was born with Marfan's Syndrome:
"The long, sinuous, hyperextensible fingers of his left hand gave his fingers an extraordinary range of motion and freedom of independent movement on the fingerboard, while the laxness of the wrist and shoulder joint of his right
upper extremity gave him the pliancy required for masterful bowing. The evidence for this hypothesis necessarily is inferential, but, I believe, convincing and even compelling.
"The range and independence of motion of the fingers was so extraordinary that it was widely suspected that he had undergone a surgical procedure to cut the bands that connect the tendons... However, it is unlikely that a talented musician at the dawn of his promising career would have so risked the safety of his hands, particularly given the primitive state of the surgical art at that time. Much more probable was that the facility was inborn."
Schoenfeld goes on to note that "the clinical features of Marfan's Syndrome were not even described until 1896, more than a half century after P.'s death," and that those symptoms would most likely not have been detectible to early 19th century medicine, anyway. "We cannot expect,
then, to find descriptions of these telltale complications of Marfan's Syndrome in P.'s life, even if indeed they had existed." However, Schoenfeld points to P.'s loss of his voice toward the end of his life, noting that it may have been "the hoarseness and aphonia caused by recurrent laryngeal nerve paralysis brought about by an expanding aneurysm of the aortic arch."
http://tafkac.org/celebrities/paganini_stories_myths.html

By the way, it might take a bit of fingering work, but even an average sized hand could theoretically work a 3-octave spread between the four strings of a violin. The 5th harmonic of Eb on the D string (that's Eb and a lightly touched G) would give a high G. D would be played on the A string, and either G or B could be played on the E string. The G string, of course, would be played open. Again, it would take a bit of flexibility on the part of the player, but the chord could theoretically be played. (G minor might be a bit easier than G major.)

It should also be noted that sometimes Pagainini liked to play with the tuning of his strings, for showmanship purposes and (according to him) to savor the sonorities of flat notes.

Another minor note: Stravinsky wrote a violin concerto whose main motive was a 2.5-octave chord played on three strings. The missing string in his case was G. The chord he had looked difficult on papaer, but is in fact quite easy for violinists. Even stopping the G string at A could produce a great 3-octave sonority with such a configuration.

By the way, another fingering that would give a 3-octave spread between the four strings involves more of hand crunching than a hand spread: From the lowest string to the highest - low A, F with the harmonic A (thus giving the fifth partial and A three octaves higher than its neighbor), open A and a stopped A on the E string. The fingering is quite uncomfortable for most violinists, to be sure, but Paganini was by all accounts more skilled than most violinists, and even to the beginner it is possible as an excersize in finger dexterity and versatility.