About all the truss manufacturers and pole barn companies use computer applications that design their products, provide material lists, etc.
You can find a lot of good general information on the internet, but there are so many variables that it would be impractical for a company to show accurate dimensions for all possible combinations.
Your term "storage truss" is very general. It could mean anything from a truss designed for storage of few boxes of Christmas ornaments under a low roof to a good sized room with tall ceilings used to store heavy machinery.
Dimensions of trusses will vary not only with the desired interior height, but also with the dimensions of the lumber used to build them. Trusses spaced 8' on center will require heavier lumber than trusses spaced 12" on center. Wider storage areas and those designed for heavy loads will require heavier lumber than for narrow storage areas and minimal loads.
If you desire to insulate your building at some point in time, you need to plan for where any insulation will be located. It's difficult to install 12" of "attic" insulation when your trusses are built with 2" x 6" lumber. Ventilating the "attic" may also present a problem.
Don't forget that you need access to the storage area. A small trap door or pull down stairway may work fine if you are only storing a few boxes of Christmas ornaments. Something different may be required if you want easy access for storing large, bulky, heavy items.
If the 20' height limitation is other than self-imposed, you need to determine where measurements will be taken. This could be the top of the skirt board, bottom of the skirt board or ???
