How can I make the image in blender the height and length I want it to be?

i want to do this without stretching out the picture because i want the picture to look perfect.

Try
http://blendernewbies.blogspot.com/2010/01/tip-match-image-plane-aspect-ratio-to.html

i meant the camera sorry

In the render settings (F10) in the format panel you can set the render width and height settings in pixels (sizeX, sizeY). You can also change the camera lens by selecting the camera and in the Editing (F9) camera panel by changing its focal length or degrees by selecting the D symbol.