I do alot of my own brick scribing for my buildings, I kind of just wing it, but I do use a rough measurement as a guide. 2mm wide x 4mm long should work, hell I usually end up with different sizes, so it's not going to really matter too much.
As for the interior, go with whatever your imagination comes up with. If you want to do a plaster look for the interior, try using an off white color, say a Antique White craft paint, then do some dark washes, and pastels, etc.
If you go the wallpaper route, there are downloadable designs, or you can buy some. I buy some photograph album scrap paper sets, and use them. A word of caution. (And I found this out the hard way) if you use a paper product, I wouldn't add a wash to it, I did and lost all of my designs! I would go with straight pastels in this area.
Cheers,
Kelly
"There you go with those negative waves again!"