aww
im starting ma 2nd year at 6th form and its in quite a large school.
it wont be as bad has you think and the 6th form probs wont be that big compared to the rest of the school
also something i found out is that 6th forms are quite close and people look out for and support each other
also 6th form classes are quite small so im sure you will make new friends quick!
dont worry about it you will be fine