I made a load of bricks yesterday, about 4mm by 2mm.
Rolled out a pack of FIMO onto a big ceramic tile using a wine bottle then just cut in the lines (no bond work, etc).
Immediately after baking the FIMO is still a tad soft and so a little bit of lifting and gentle scrunching will snap the individual bricks apart.
The sensible thing to do is to use FIMO the colour that you want the bricks to be ...... I didn't as I just used what I already had.....
I can't tell you how many bricks a pack produces as I didn't count the lines before separating them!