Do a search on Youtube. There is at least one specifically on how to do this and I believe "Picture in Picture" is even in the title. I don't have time to look right now. If you don't find it let me know. I'll get it for you tomorrow afternoon.
VEGAS Movie Studio 15 (basic version) does not have the Picture in Picture effect plug-in that is available in VEGAS Movie Studio 15 Platinum. The online Help (F1) in VMS 15 advises that you can use the Cookie Cutter effect to create picture-in-picture effects or to crop your video.
Eduardo, first off, whatever you do, don't pay any attention to what I say. Follow vlmast's advice. He know 300x what I do about Vegas Movie Studio (and that is an understatement). When I told you about the YT videos I didn't realize that the Picture in Picture is a plug-in available in Platinum not Standard. Most of the videos use the plug in.
But there is this:
This, I think, will help you. He uses the "Track Motion" button to resize one track.
That video is from a VMS Platinum version even though it is not clear from the title. Unfortunately the basic versions have less options that the Platinum versions. Even the VMS product pages sometimes erroneously show VMSP videos.