This is a deeper look into some of the statistics I've measured over the years on various stock water blasters (The first part can be found here). These stats are pulled from the iSoaker.com database which is queried by using the iSoaker.com Product Listing.
Perhaps what should be noted first and foremost is that the majority of water blasters in the iSoaker.com database are either of the Super Soaker (now Nerf Super Soaker) and Water Warriors brands. There are also Speed Loaders, Storm Guns, X-Stream, and some others types, but the numbers found here cannot and do not capture the sheer amounts of random, no-name brand or "dollar store"-type water guns that can be found in stores. While I don't have definitive numbers on all of those, in general, the cheaper knock-offs tend to be mostly squirt pistol sized (less than 20cm long) though there are the occasional non-major-brand water blaster that ends up being fairly large (there were some Hulk and Spiderman branded water guns released a few years ago that were probably longer than a Super Soaker CPS1500, but performed quite poorly).
Moreover, these stats can't be used as any way to forecast what future water blasters would be like. However, these stats do allow us to look at various trends found in the major brands and permit us to better gauge how well a particular water blaster model performs when compared to other blasters of a similar nature. Additionally, while we don't have numbers for every single water blaster in the marketplace, the trends found in the major brands likely apply or explain things that would be found in other brands as well.
A Look at Lengths
The graph below shows the number of water blasters of a particular length as measured in the iSoaker.com database.
As can be seen from the above graph, water blasters come in a variety of lengths ranging from ~5cm to greater than 95cm. There are some notable clusters of lengths, though, particularly around the 40cm-45cm (~1.5') length and the 59cm-60cm (~2') length.
This clustering is likely more intentional than not, particularly since water blasters must fit nicely into the dimensions of store shelf space. With common shelf widths being on the order of 90cm (~3') to 120cm (~4'), having blasters that package nicely into these shelving limits allows better use of store presentation space. Smaller blasters (<25cm) tend to be found on pegs or hooks as opposed to being placed on shelving. There is the grey zone between 30cm-40cm in which sometimes blasters are found on shelves while others are found hanging from hooks.
The longest blasters (>85cm / >~3') are from the height of Super Soaker popularity with blasters such as the Super Soaker Monster XL and Super Soaker CPS2000. We have not seen a water blaster approach that length in the recent years with the exception of the Nerf Super Soaker Hydro Cannon that measures in at 75.5cm (29.7").
Simply put, if a blaster is longer (larger), it ends up occupying more shelf space so it had better sell very well to justify the space it takes away from other products. With the recent pull of consumer spending dollars into other things (e.g. video games, mobile apps, cell phone cases, etc.) as opposed to water blasters, retailers appear to be opting to stick with smaller-sized water blasters, saving space for other products to sell. Even at the height of the summer water blaster buying time (tends to be ~July 1-4), I recall the days when I would walk into a store and be presented with a huge wall of water blasters. In recent years, I'll be lucky to find that a couple of sections are devoted to water blasters with the remainder of the wall given to other summer toys from inflatable pools, beach toys, and other similar non-water-blaster water toys.
To make a long story short, while water wars are still well enjoyed by pretty much everyone I know, their size and frequency has declined. However, I do not consider this a losing battle and I still feel we may see improving water blasters in the future. With Hasbro Inc. now having significant competition in the water blaster realm with Buzz Bee Toys Inc. and a growing shift to promotion of more organized water wars, there appears to be a promising future ahead. Of course, time will tell.