Flash MX 2004.
bitmapInstance.vPixels
Read-only property; an integer that represents the height of the bitmap—that is, the number of pixels in the vertical dimension.
The following code gets the height of the bitmap in pixels:
// Get the number of pixels in the vertical dimension.
var bmObj = fl.getDocumentDOM().selection[0];
var isBitmap = bmObj.instanceType;
if(isBitmap == "bitmap"){
var numVerticalPixels = bmObj.vPixels;
}