Calculate report's width and height at runtime

Hi all, 

i must export report to png image, my report has three bands (Title, detail and summary).

Title height is 290 pixels.

Details height is 270 pixels

Summary height is 460 pixels.

The total height is 1020 pixels.

In according to the contents the total eight may be greather than 1020 pixel, how i get in my java program the real height at runtime?

If i write in java: 

jasperDesign = JRXmlLoader.load(inputStream);

jasperDesign.getPageHeight() -----> this return 1020 and not the real height at runtime