The span is 100 feet. It's sided for the same reason it's covered - to proctect the structural members/trusses from weather
True, but even more important to those who originally built them was that the solid sides kept the horses from being spooked during the crossing and getting out of control.
I used to run in an annual Covered Bridges Half Marathon in Quechee, Vermont. It was a beautiful, unique venue with four covered bridges, two of which we ran through. It was an exhilarating experience to race through the cool, dark atmosphere of a covered bridge.