Sizing to real world objects

Hi There!
I’m relatively new to Blender, having just downloaded the program a couple of months ago. I’ve watched some of the tutorials and am trying to get through the manual to familiarize myself with the interface.

I am into train simulation, having been involved firstly with MS Train Simulator and now Railworks 3. On an exploratory excursion, I’ve located a couple of old artifacts along the rail line that I used to work on, the Canada Southern (NYC/PC Canada), these being an old concrete whistle post and a mile post marker. These artifacts are roughly 4ft x 1 ft x 2 in. I’ve dabbled a little first with GMAX and more recently with 3D Crafter. These 2 programs allow you to set the grid size to various units, i.e. cm, metres, in, feet etc to build models to scale. The only grid sizing I’ve been able to find for Blender relates to the number of pixels per grid box. Is there a way to specify the grid in cm or metres?

Go under the scene tab and go to units.