Using premiere pro, is it possible to change a movie size to fit 720/480? That is, imported clips of differing sizes.
Or is there another program that can do this?

It should be able to do this. Set your project to 720x480 and
import your footage. You might need to fool around a bit with
the project settings or footage properties. I don't have PREM
Pro here so I can't check it for myself.

I have recently had to use a MPEG1 in one of my productions that went to DVD for a client of mine. I strated a new project in Premiere (v6.01) and imported the MPEG1 file. The I exported the whole file to a DV AVI and the resizing was done with no problem. Then I could do whatever with the resized file... edit it grade it and etc.

But keep in mind resizing from small to big will always not look crash hot, and this applies to all raster graphics too!

Hope this helps.
Jack

Maybe I did something wrong. Seems like I did that, but the new file just placed the smaller film in a larger format rather than increase to fit the new size.
Gonna try it again and see, thanks for the replys guys.