How to get size of a rotated rectangle

Possible Duplicate:
Calculate Bounding box coordinates from a rotated rectangle, Picture inside.

I have a rotated rectangle, So how do i calculate the size of axis-aligned bounding box for the rotated rectangle in 2D Coordinates?

Attach Image http://img88.imageshack.us/img88/503/rotp.png

i know x, y, o (angle) but how do i get a, b

Thank you