Note that you can't have more than one frame size for the sequence - you can only scale the clip up or down within the sequence frame.
For a simple edit transition, this can be done easily by cutting the clip into two clips. Select the first clip, and select Clip > Video Options > Set to Frame Size. Then select the second clip and add the Auto Reframe effect.
If you want the transition to be more advanced, for example, if you want to animate the scale of the clip, you can do this by adding keyframes to the Motion effect in the Effect Controls panel.
Cheers,
Paul