How to get comp height and width.

myComp would be a reference to your comp object. You might get it from the activeItem if your comp is open and selected:

var myComp = app.project.activeItem;

If not, you'll have to loop through the project items until you find a comp item with the name of the comp you're looking for (not tested, but should be close):

var myCompName = "Main Comp";

var myComp = null;  

for (var i = 1; i <= app.project.numItems; i++){

  if ((app.project.item(i).name == myCompName) && (app.project.item(i) instanceof CompItem)){

    myComp =  app.project.item(i);

  }

}

if (myComp != null){

  var w = myComp.width;

  var h = myComp.height;

}else{

  alert("Can't find comp " + myCompName);

}

Dan