That depends on how much truss depth your site can tolerate. From a layman's view, the truss is really no more than a large beam made entirely of smaller peices. On the top and bottom of the truss are chords that are in tension or compression depending on the support and loading conditions. This is similar to the extreme fibers of beams in tension or compression. As you may recall from Mechanics, the deeper the section the higher the second moment of inertia and the more load that can be sustained. Or for smaller loads, the greater the span lengths.
Just recently we had proposed a several hundred span deck truss bridge. It had several supports so it was indeterminate and it was also shaped as an arch.
In summary, we need to know more about your situation.
For information on long span structures with stiffening trusses search the web for the Akashi-Kaikyo bridge at Kobe, Japan. This bridge is a three-span suspension bridge that is over 3100 meters in length with a center span of over 1900 meters.
I believe it is presently the world's longest suspension bridge. However, there is another bridge project in the works that will dwarf the Japan bridge - it will be located in the Messina Straits of Italy.
