Need to make a test run on packing for Vegas

I need to pack my bags for a test run on what I want to take with me to Las Vegas. I have a good idea of what I want, but I am not sure about what all I am going to be taking with me. A test run on packing will give me an idea on what kind of room the things I am pondering taking will require. From there I can add or subtract things from my suitcases until they look right. The important thing is to make sure that I leave room for anything that I might be bringing back, I don't know how many things I'll end up with from the Expo and stuff, so I want to be sure that I leave some room. I really don't want to have to mess with more bags than I have to.