You have to roll your own but it's easy enough.

if (app.project.selection.length == 1) {

          var sourceItem = app.project.selection[0];

          app.project.items.addComp(sourceItem.name + " copy", sourceItem.width, sourceItem.height, sourceItem.pixelAspect, sourceItem.duration, sourceItem.frameRate);

} else {

          alert("select one item in project window");

}

Yes, I was just giving you an example to get you started. I'd generally suggest checking out AE's CS3 scripting guide and the Javascript Tools Guide pdfs for documentation, and posting questions along with your code so far when you get stuck. You can study the various examples on here and places like aenhancers.com to get a better understanding of the syntax.

And one minor point, expressions (javascript that controls a property value) and scripting (javascript that controls After Effects) aren't exactly the same thing. In this case we're talking about scripting, not expressions.

Here's an example that should be closer to what you want:

var frameRate = 30;

var maxDuration = 10;

var maxWidth = 10;

var maxHeight = 10;

var pixelAspect = 1;

var x,y, curItem, thisItem, theComp;

for (x = 1;  x <= app.project.rootFolder.numItems; x++) {          // loop through items in root folder only

          curItem = app.project.rootFolder.item(x);

          if (curItem.selected && curItem instanceof FolderItem) {          // if it's selected and a folder

                    for (y = 1; y <= curItem.numItems; y++) {          // loop through the folder's items

                              thisItem = curItem.item(y);

                              if (thisItem instanceof FootageItem) {          // if it's footage

                                        if (thisItem.hasVideo) {          // if it has video

                                                  maxWidth = Math.max(maxWidth, thisItem.width);          // record this footage width if largest

                                                  maxHeight = Math.max(maxHeight, thisItem.height);          // record this footage height if largest

                                                  pixelAspect = thisItem.pixelAspect;                    // record aspect ratio

                                                  if (!thisItem.mainSource.isStill) {          // if it isn't a still record frame rate and duration if longest

                                                            maxDuration = Math.max(maxDuration, thisItem.duration);

                                                            frameRate = thisItem.frameRate;

                                                  }

                                        } else {          // must be audio. record duration if longest

                                                  maxDuration = Math.max(maxDuration, thisItem.duration);

                                        }

                              }

                    }

                    theComp = app.project.items.addComp(curItem.name, maxWidth, maxHeight, pixelAspect, maxDuration, frameRate);          // add comp with max values and folder name

                    for (y = 1; y <= curItem.numItems; y++) {          // loop through items in folder again, adding them to comp if they are footage

                              thisItem = curItem.item(y);

                              if (thisItem instanceof FootageItem) {

                                        theComp.layers.add(thisItem);

                              }

                    }

          }

}