Resizing video or image clips is done using the Scale option under the Effect Controls on the top left window. Decreasing the scale value will make the clip smaller while increasing it will enlarge the clip. To quickly scale up a video to fill the scren. (Say you had a 720P video in a 1080P project) Right click the clip in the timeline and click “Scale to Frame Size.”
For a tutorial on working with scaling as well as other clip properties, click the link below.
Lynda Playlist: DLL Adobe Premiere Keyframing: