Depending on the availability in your market, I suggest using 2cm (.75" thick) granite with laminated edges (1.5" thick finished product). Build the frame as Kevin54 suggested, using cement board as your sub strength. A 72" run of granite is HEAVY either way, but using 2cm material will be easier to support, being that it's around 150 lbs lighter. The cost between 3cm and 2cm with laminated edge work will be about the same, but then again, that may depend on your market and the fabricator/installer that you choose.
Build the countertop to be an inch or two wider than window casing (if any).....it will give it a nice transition from countertop to trim work above it. Most pass-through windows, bar tops, plant window sills, etc, are not normal countertop depth. In my opinion that would be more of a table attached to the wall, rather than a bar ledge. If it were me, I'd do something in the 12" to 16" range. Less depth on the granite will allow you to use corbels for support, rather than table legs that extend to the floor.
I'll dig through my pictures and see if I can post some examples of ones that I have done.