HPRoxMySox, the SortingHat's PM box is extra large so that all those quiz requests and answers can fit in without the box exploding. If you receive a message that the receiver's PM box is full, double check that you spelt the receiver correctly: SortingHat - nothing else. And without a space character in-between the two words.
Also, everyone, please be a bit patient if it takes a few days until you either receive your quiz or an answer where you've been sorted. We mods manage the SortingHat account and thus have to check over the quizzes on top of RL and our other modding duties. You eventually will get a quiz as well as the results.
And like inspirations said: All our houses are fun to be in and have lots of great people, so no matter where you end up, you'll be welcomed with open arms. Just don't be shy and join the inhouse chatter/activities and you'll make amazing friends. :)
Nicole's edit: I agree and may I add that I about had a heart attack when I was sorted into Slytherin and now . . . if I were given the chance I would NEVER leave my Slyths!