I'm a newbie too, but I'll give a little input from what I've seen. A 20 or 24 foot span is doable if the timbers are sized correctly as well as the joints. Ask yourself what the intended purpose is and determine the size you really need. Whether the frame is 24 feet wide or 32 feet wide, it's roughly same amount of work to build. I'm assuming a king post design and that you want to clear span your width. So if you needed or wanted more space, consider that it's just as easy (almost) to up it to 32 or whatever. The tie beam does not have to be continuous with a king post, thus allowing you to use 2 16+ foot pieces for your tie beams, joined to your kingpost. This was the design we used in my recent timber framing class. Our clear span was 24 feet, but a previous workshop did a 32 foot span in the same style.
I'm now wondering how you'd connect it to the other building. It would seem easier to connect a smaller addition to a larger frame as you can just cut mortises in the tie beam of an existing bent and run your girts into them between that bent and the new addon bent. You could also easily add a same size addition onto an exiting bent the same way. Seems a little trickier to add a larger addition on. you may have to remove part of the roof and side walls of the existing and remove the bent (if you need to start the larger addition right at the rear of the existing. Hmmm, that would be somewhat wasteful though...Maybe someone will have a better idea.