From "Your pants are large", we don't know if it is one pair of pants or multiple pairs of pants. If such a sentence is proposed which could be honestly interpreted as either singular or plural, both "Ton pantalon est large" and "Tes pantalons sont larges" should be accepted.
"pantalon" = a single pair of pants or just pants "pantalons" = several pairs of pants
I've never heard any one say "Those are nice pant you're wearing!". It's "pants" even in the singular context. If you want to play it safe in DL, just say how many pairs of pants there are -
"Ton pantalon est large" = your pair of pants is large
"Large" may be big, broad, wide, baggy, generous, open (in the sense of large), and can also specifically refer to the width of something. As a noun, which this is not, it can even refer to "the open sea" but I'm not suggesting that here, of course. For a given application, not every one of these English words may be used with the insistence that it could mean that. So first consider the picture (not an English word, but a picture) in your mind when you are getting ready to choose the right word, and then choose accordingly. http://www.larousse.com/en/dictionaries/french-english/large/46205