What is context window size?
A context window applies to the number of words you will use to determine the context of each word. Like of your statement is "the quick brown fox"  a context window of two would mean your samples are like (the,quick) and ( the, brown). Then you slide one word and your samples become  (quick, the ), (quick, brown) and (quick fox) and so on. I would suggest reading up this word 2vec tutorial to understand the training method and terminology.