Kalonian hydra in combo with xenagos god question

[rr260341]

If I swing with {Kalonian Hydra} and have xenagos (god enchantment creature) on the board does it swing as a 12/12 or a 16/16?  I believe it's 12 due to xenagos trigger occurring at the beginning of combat but wanted to verify.  Thanks.

[Kaworu, the Fifth Child]

12. It goes like this.
Enter combat
4/4 plus 4 from Xenagos (8/8)
Attack with Hydra
Gets 4 counters due to Xenagos not giving counters(12/12)
12/12 at damage