If there was a 10″ span for the balsa wood bridges then the bridge would need to be more than the 10″ (11″ or 12″ would be a start). The height of the bridge would depend on how much weight need to be held. The more height you had, the more weight the bridge could support.
I think 10 24″ pieces would be plenty to build a sturdy and supportive bridge. You can always cut the pieces to create your design and make it the way you intended it to be. But the more wood you had the better and a stronger your bridge would be.
With a top truss design you can have a stronger structure with less material used. Adisadvantage of that is you waste more materials. A bottom truss bridge would weigh down the bridge more than a top would and it would use more materials to build it.