Glen;
Here are a few standard measurements for 1/35 scale to get you started. You would do well to find a copy of Roy Porter's book it is full of useful tidbits.
The first is the "true" or real life size the second is the scale size.
Door - 6'6" = 57mm
Window - 4'0" = 35mm
Brick Length* - 9in = 6.4mm
Brick Width* - 41/2in = 3.2mm
Brick Thick* - 3in = 2.1mm
* Brick sizes are subject to small regional and national variations which are not, however, apparent to the casual eye. The sizes above may be regarded as including one course of mortar.
Now as far as your idea with DAS modelling clay. It works quite well too. What I do is this. I first make a form of the building that I want to portray using thin plywood. I cut out the door/window openings or cut in any "missing" area that has resulted from battle damage. Once assembled, or at least cut into subassemblies depending on the complexity I drill or score the surface creating small pockets over the entire surface this allows the DAS to adhere better. I cover the surface evenly with DAS and begin sculpting my brickwork with various tools. Most importantly is that your bricks appear straight and even. Once you're done scultping allow the DAS to dry and paint the next evening.
To make your own bricks.
Take a sheet of styrene (any size will do) however it should be of a decent thickness to prevent bending. Glue strips of 2mm thick styrene spaced every 5mm (this includes the 2mm thickness of the strip) until you have covered the surface. Once done and it has dryed with a razor saw or similar cut straight across (intersecting the now horizontal strips) at every 6.5mm. What you have now created is an easy mold for your DAS brickworks. Now all you have to do is roll out the modelling clay and fill the troughs between the strips. You "cut" the bricks with sewing thread stretched across the mold and pulled down into the cuts made every 6.5mm. Depending on how big your mold is you should be able to generate a lot of bricks in a very short time. When each batch is made turn your mold over and they should pop out and your ready to begin again.
I hope this helps. Sorry I took so long in getting back to you. I had to retrieve the book from a friend.
Cheers;
Gregory