Help with movie rental

I rented LOTR. Half way through the movie, the phone rang and I closed the app. I knew after that I had to redownload it. This time, it said I didn't have enough space. My two questions are why didnt it tell me the first time (I wouldnt have downloaded it if I knew I didn't have the room) and is there a way I can only download the part I need to watch? Now I wasted 5$