Ok, I found it out by myself, it is 1514b.

I set up two length matches in the PREROUTING chain in the mangle table, one
set to match on packets >=1500b and one >=1501b. The latter rule never
matched so I thought that the max packet really was 1500b. However I then
decided to put all packets matching the first rule in a separate class.

>From there the answer was obvious, I simply divided the byte-count with the
packet-count and voila, I got 1514. So I guess the length match doesn't
count for the 14b's of ethernet encap.

This led me to change another thing in my setup, I no longer match small TCP
packets by matching them by <=64b but rather <=50b.

Well... this prolly seem real newbie like to most of you but hey we all are
at first.

// Daniel