The first question is, how do you want to make it bigger? I fyou want a bigger water tank I would up the weight rating of your trailer. I prefer two smaller water tanks to one big tank. The reason is that when I recapture water to keep the enviro nazis happy, I can put fresh water in one tank and clear water in the other. My trailer is 12 foot long and about 90 inches wide. It is at the maximum legal width. That way I can mount my two tanks side by side. The pressure washer is in front of the tanks, with heater perpindicular to the sides. There is room in front for about 6 chem buckets, a 35 gallon chem tank, and a tool box on the side. On the back I have a Vacuum hose reel, a pressure hose reel, and a feed hose reel. There is also a vacuum recovery tank. The vacuum system is loose right now, but I am going to mount it on top of the water tanks. The generator for the water recovery system is going to have tofind a home on the trailer as well. I am just trying to figure out where to put it. That is a system that works for me. I store wands between the pressure washer and the water tanks.
I don't like an excessively big trailer for a few reasons. The biggest is that I have employees and they love to put their garbage on the deck of the trailer and leave it, so there is less room for garbage.