Breaking ciphers and certainty

I have been exploring an encryption algorithm - and now I want to know if it could be considered 'robust'. Best case scenario, I sell it to the NSA or CIA or something similar. But I also have very little idea of where to post or send samples for valuation. I have already tucked a large sample onto my Facebook page, but with no apparent interest raised. It also raised a question for me: How large a sample would be needed in order to be 95% certain of being able to break an encryption method? And - if this is not the best audience for such a question - who or where would be?