Lateral stability, especially during construction, would be a big reason to go long if possible.
If the truss is uniformly loaded, to size for gravity loads, determine the tributary width, multiply the live and dead loads by the square footage bearing on the truss (trib width x 26' x (LL+DL)). Distribute the load uniformly on the nodes of the loaded edge. The members are solved as either columns or columns with bending.
"Wood Engineering and Construction Handbook" Mcgraw Hill, has a good description of sizing a 50' span heavy timber parallel chord truss. Interlibrary loan might be able to get it, chapter 6.
