Okay,
With this one I need some help.
Last weekend I was in my local toy shop (you know; shopping for 06/12). I came across the railroad section and saw something interesting. It was a little plate, painted grey and ready to scrape some forms. Sorry, don't know the english word for isomo.

I used my graphite pencil to scratch a cobblestone road and used the ivory black wash. But I haven't got a clue on the size if the individual stones or the form.
Here you can find a picture of a little piece of plate. The second picture is only to demonstrate the scale.
Have I used the correct size and form or should the stones be more of the same size and form.
Any help is welcome.

Bep,

Your styrofoam (isomo) project looks good.

Building your own streets / walls is a bit of a learning process. Through trial and error, I discovered that both square-shaped and rounded stones can work well if you try to imitate natural stones. Just one thing you need to look into : natural stones generally don't make inside corners. This means you generally don't get "L"-shaped stones.

To illustrate, I made a small drawing. The stones on the left are round or square, but they don't have inside corners ; only outside corners. In the drawing on the right, I added inside corners : "L-shaped" stones. I exaggerated a bit, but you'll discover that L-shaped stones feel unnatural.

The size isn't really important : you can use small and big size, even mix them. In general, stones are not larger than someone's foot... so you can use a soldier's foot for reference. Flagstones do get bigger, but please note that those have pretty clear cut edges : not the soft curvy edges of the natural stones in your drawing.

Feel free to PM me on this...