Are you serious? Do you mean you came across with the video with incorrect aspect ratio (compresses or stretched in one dimension, so proportions are mangled) and want to fix it on-the-fly in ASP.NET? Why?
Just in case: you're not talking about the video screen size which is quite usually a subject to change by the user on the client size. You're talking about aspect ratio, proportion between vertical and horizontal sized of the frame. This thing is supposed to be fixed when a movie is mastered or re-mastered. And it's supposed to be correct in the video source before it is delivered to the Web user.
Better fix the problem by re-sampling the video off-line. I usually use VirtualDub, see http://en.wikipedia.org/wiki/VirtualDub
] a free application under GNU General Public License.