So many times the answers are usually the same; read up on what your truck's payload is; what its towing capacity is and find out EXACTLY what your trailer weighs empty and start adding up what you're putting into it.

Then, consider where you'll be towing and what type of terrain. Mountains like the Rockies? mountains like Appalachia?  Or flat places like so much of the midwest. High speed roads or bumpy 2-tracks? High temps like the southwest are other considerations - it affects your tow vehicle and your livestock.

Want to lighten your load? consider aluminum for large trailers; consider no ramp; consider no enclosed tack. For those in the SW, where there's little rainfall, the stock trailer configuration - with slatted sides, is another way to lighten the load and provide air to your horses.  Consider an alum or fiberglass roof to further lighten the trailer.

There's many considerations. Also - how many gallons of water are you hauling? not just the clean water, but what's in your dump tanks? Find places that you can fill up and dump out near your destination to avoid hauling any excess weight.

Load as much as you can in your trailer to lighten your truck's load  - let the trailer carry it and then figure 10 - 25% of that weight on your truck, rather than all of it being carried in the bed of the truck.

I'm not saying to go to the max in loaded trailer weight - but there are ways to change the mix around so that the weight is where it should be.