The last flat I did was just under 5 lbs, or about 2 1/4 KGs I guess.
Including the fat cap, at the thickest part it was roughly 2 inches thick....after cooking. It probably lost 1/4 inch during cooking.
Not knowing where you purchased it I will say this, for me it makes a big difference whether I get my flats from a real meat market vs. a grocery store. I made the mistake of buying a flat from Safeway (West Coast chain) and it was much thinner, not very uniform in thickness, and had too much of the fat cap trimmed off.
A good brisket flat will be somewhat uniform in thickness, i.e. not much taper at the ends. Here is a link that not only has tons of good info on prepping and cooking briskets, but will give you an idea of what a good flat looks like.
http://www.virtualweberbullet.com/brisketselect.html