I find a little bit at Music Basics (http://www.musicbasics.com/howtodetrigs1.html) under the title "How to deterimine the right size of viola to buy."
Viola comes in 4 different sizes: 16", 15", 14", and 13". 16" size being the biggest and 13" size being the smallest. All adults, regardless of their size, use the 16" viola. There are 2 other less common sizes, 15-1/2" and 16-1/2". 15-1/2" size is for people who are in between 15" and 16" sizes while 16-1/2" size is for people who wants a little more sound volume out of their viola.
They give the following "dimensions:"
16" inch viola, 26 inches long 15"inch viola, 24 1/2 inches long 14"inch viola, 23 inches long 13"inch viola, 21 1/2 inches long
For comparison, their "How to … violin" gives:
Violin comes in 8 different sizes: 4/4 (also called full size), 3/4, 1/2, 1/4, 1/8, 1/10, 1/16,1/32. 4/4 size being the biggest and 1/32 size being the smallest. All adults, regardless of their size, use the 4/4 violin. There is another uncommon size, 7/8, usually used by female professional violinist who wants a full-size violin sound but whose hand might be a little small for the full size violin. So violin makers would make violins just a little smaller than full size to accommodate these players.
With the following dimensions:
4/4 (Full Size) violin, 23 inches long 3/4 violin, 22 inches long 1/2 violin, 20 inches long 1/4 violin, 18 1/2 inches long 1/8 violin, 16 1/2 inches long 1/10 violin, 15 inches long 1/16 violin, 14 inches long 1/32 violin, 13 inches long
They list a fairly wide range of violin strings for sale, but do not list viola strings.
Woodwind Brasswind (http://www.wwbw.com/Department/?d=8&dd=954727718&source=msn) does offer viola strings, but doesn't give any info about what makes them different form violin strings (except the price). You can go directly to their strings at WW&BW viola strings.
String Works (http://www.stringworks.com/) has an impressive selection of instruments, but no information on what makes a viola different from a violin. The do have an interesting entry to their "String Works University" that gives an apparently knowledgeable survey of "stuff every fiddler should know(?)" String Works University doesn't contribute anything to this question, but is an interesting "quick read" that might be useful with students.
The American Viola Society has a site, but I note that their "hitmeter" showed me as the 3,837th visitor since last November. Not exactly a hotbed of excitement (unless the meter's broke)? I didn't explore the site.
The best place I found with my brief survey, where you might be able to "dig out" something of interest, was at The Strad monthly magazine site. The site appears to offer a link to browse at least some back issues, and I've picked up a few newsstand copies, so I know it's a good mag.
There seems to be a lot more internet interest in Viola da Gamba and Viola d'Amore than in "conventional" viola. Unfortunately, in this area I found some sites that I'll be compelled to go back to for another look; but they don't contribute anything useful to the current question.
The few decent pictures I've hit suggest that the modern viola may tend to have a slightly narrower "upper bout" than would result from directly scaling a violin - possibly to prevent interfering with the bowing arm. I can't state this as a conclusion, it's only suggested by a few "representative(?)" pictures of mixed quality.