[Abhishek R]

Some software, like Adobe Illustrator, Inkscape, and GIMP will let you set the dimensions of your canvas to inches, then you can work off of that. It's like CAD for images, literally.

I believe the last two I listed are free, last time I checked. Inkscape especially is particularly nice, for a free product, though I personally prefer Macromedia Fireworks/Illustrator.