Agreeing with the previous posters, there are as many different dimensions for tanks as there are tanks. Personally, we have two of the used totes inside the aluminum cages. They work very well for as little as we've ever used them. In fact, have not used once this season and our business has expanded by 85% this year.
As far as a trailer goes, I would highly recommend a dual axle for hauling water & equipment. Electric brakes would also be a good thing. Water weighs 8.34 lbs. per gallon. That's a little over a ton per 250 gals. This, combined with equipment, materials, etc., adds up fast. Also, when the tank is say, half full/empty, braking becomes more of an issue. The water will act as a wave pool, pushing & pulling you as you accelerate or deccelerate quickly. If you consistently need to haul water, the larger trailer with dual axles will last far longer than a smaller, single axle.
Perhaps the more important question is how much business will you do, how long do you intend to stay around, and can you get by with less without throwing good money to bad? We tend to stay very conservative when making major equipment purchases, but will definitely spend whatever makes sense for the right piece of equipment. For example, we purchased a "hardly used" 6' x 10' single axle trailer 1 1/2 years ago at a good price. It has served us well, but didn't give us the flexibility or working room we had hoped. We have since converted this trailer to accommodate our post-construction cleaning set-up and purchased a 14' tandem axle with elec. brakes for our wash rigs. Fortunately, the money spent on the first trailer was not wasted, but we discovered only by using it over a period that it didn't offer us the flexibility or efficiency we needed.
My point being.......getting advice from this forum will help make your decision. You're the one that has to figure out how much a new trailer is going to help you make money, and how much money do you "really" need to spend to do what you do. Good Luck