the new 13 is exactly the same length as the original AICS pattern 10 round mag. The 10 round MDT mag is about 3/4" shorter, and slightly longer than an AW.
I've shot the prototype 13 over 1000 rounds including match conditions and it is flawless with a 308 in a Remington 700 action. It's also ran fine in a 6.5 CM but I have no personal experience. It is identical to the 10 round magazines (on far right in the picture) except for length, and we have been using these for over a year now.