Hello Miloslav
Here's what I have found (thanks to Google) :
Rations per case: 12 (2 meals each of 6 varieties)
Case dimensions in centimeters: 39.5 long x 23.2 wide x 24.5 high (15.5" x 9.1" x 9.6")
Case volume: 22.4 decimeters (3.1 cubic feet)
Case weight: 7 kg (15.4 lb)
from
HERE These specs apply to a "civilian" MRE case but it appears to be very similar to the "military" one :

HTH
Frenchy