As far as I know, the reason people always say no more than 5s is because it gets "too spammy", but frankly, we all know someone is going to bitch about spam anyways...whether it is 7s, 5s, or 3s.
For some siegers, its just built into their nature to bitch about something. At least with 6s+, most people are getting the chance to have fun playing the game they love while the ones who were going to bitch anyways are doing their thing.
I've always been in favor of 6s+, but I don't recommend 7s on a 14 person server. If one person has to go AFK or leave (which is highly likely with so many people) then it is pretty disruptive to the game to keep yelling for someone to go spec on the other team. I would recommend only doing 7s with a couple of subs available, but since the max capacity is 14, you won't have that luxury.