Why does Twitter limit tweets to 140 characters? Why this number? What are pros and cons of this design?

I am developing an educational website in which students can share luthier learning tips with their classmates. It is important for me to make the posts as small as possible. However, I don't know how small they should be. How can I impose a similar limit on videos or images they upload? I was wondering Why Twitter limits tweets to 140 characters? Why this number? What are pros and cons of this design?