Agree with many others, the venue being a barn says nothing about dress code. I have been to only one barn wedding, and it was quite a dressy affair. The rustic backdrop only added to the cool ambiance.
It's too bad the couple didn't give any guidance as to what they wanted, but you could always ask them or someone close to them who is in the know. Either way, I think leggings are too casual for any "event" !