ahahah, no, no military reasons. it´s just as local organization goes, seaside falls into the jurisdiction of the navy, thus under the ministry of defense
the water is colder up north, as outside weather.
Guincho is close to Sintra mountain range and somehow this take a big toll in local conditions.
i´ll leave the explanation on how all this affect wing strength to others, not that i don´t like to understand things but there´s a point i quit trying understand what´s going on in God´s mind
on a more practical note:
if you want to ride in Guincho most probably you will need the 7m. But then this kite size is pretty useless in all other spots around.
Between the 10m and the 13m pick 1 of those 2, guess they won´t be miles apart and you´ll be able to have time in the water with one or the other