You can still post a few lines to ask for help, but I think we have all the information we need to help you.
"Circumference (in stockinette): 30cm"
That would be about 12" and while they make 9 and 12" circulars, many people find them a pain to work with as the tips are very short. However, you can use longer circs - 24 to 32" and use the magic loop or single loop techniqus. Magic loop is show on the Advanced techniques page, and SL can be viewed at One Loop (Travelling Loop Method)
If you're going to use more stitches, you can probably do one on a 16" circ.