You want a PiP (Picture In Picture) for your video I am guessing.
1. Place the video you want to resize on track 1.
2. Place video you want to have video to appear in on track 2.
3. Right mouse click on video in Track1, select Open Effects Editor.
4. In Effects Editor, select 1. 2D-3D>Studio PIP.
IF you are using PS18 (you did not say which version of PS you are using), by default, the Solo button is OFF, so you can see the layer that is below. IF you are using PS before this, you have to turn the Solo Button OFF. It's on by default. This was changed in PS18.
Now, you can use the adjustments to place your PIP, resize it, add border or drop shadow, etc..
Read this LINK on how to add a screen shot to your post.
Here is a link to a video tutorial on how to do this. It works the same from Avid Studio to PS18. PIP Tutorial.