SIZING AN IMAGE

Question

I am working on a project for photo printing and want to allow the user to feed the image dimension in millimeter, but, how should I use the drawimage() in vb.net to convert the millimeter to pixel. The image may be of any size or resolution. hence, the code may display only a portion of the selected image depending on the dimension specified by the user but should not affect the aspect ratio of the image. Please Help me out.