How to make a wmv file stretch to screen size when converting in Premiere Pro?

I'm completely new to video editing so excuse my idiocy. I have some video clips that I'm compiling into one file and encoding to play on my ipad with Premiere Pro. Some of the clips are very small, and when it finishes encoding playback doesn't fill the screen. (They are WMV files) Basically, how do I set this to stretch the video to fit the screen without having to zoom in manually?