Does a transition have to be one sentence? Why or why not?

Well yes and no

Explanation:

Yes you can transition an idea to another in just one sentence, but if you want to go into brief about a little you can do more than one sentence but I would not recommend more than 2-3sentence of transitions though

hi, I am a ballon I blown up by a young man, and given to a little girl. As the little girl was walking down the sidewalk she accidentally let go of me. As  I was floating up a bird came out of nowhere and popped me, so down I went landing on a very hard and hot surface very high up.