With all the equipment alone about 13,000 pounds for just that is what I would go with. Even if you don't roll with water all the time, you still will be some of the time which means that you have to have a truck carry it all. So you are looking for a truck that is just under the cdl, so 25,500 pounds is about right. In terms of length, it all depends on how good you can stack, and how tight you want everything to be. With one truck you will need more hose to spread out with 4 guys, but you can have them come to site in their own cars and probably save on insurance not having them all listed on the truck.
Another thing to keep in mind, truck are built with a maximum capacity, that does not mean though that you should build or plan your truck to this. I have seen many times where people have done this and have had the truck in the shop due to suspension and brake issues. So do you want your truck to be making $$$ for you, or paying $$$ to get it fixed and maybe losing customers due to not being able to do their work.
I planned and the built the truck way over what I needed, more so I didn't have to worry about scales and I could go on roads that would allow 75% of my total gvwr. Also parts are way cheaper on the bigger truck then I could have imagined. Since the truck is pretty much a highway tractor parts are available pretty much everywhere and are dirt cheap. Things I would expect to be expensive are not. I have heard stories of guys having to do brakes on 650's and having to order parts and pay $800 for all the pads and rotors. While my shoes and drums for the entire truck is half that price and I have 6 sets vs 4 sets to do.
Only draw back is I had to get a special license and yes I have to do yearly inspections, but I weighed that all before getting it. I bought the truck with 280,000 miles on the odometer, had to put a box on it and build everything in it. I needed lots of water as I can't fill on site, and now with the big truck I can run 2+ people for an entire day.
In saying all that two trucks might be better, but if you are going for one go as big as you can as it is better to have an over built truck then one that can barely handle what you put at it.
Just my 2 cents