headers don't specify a size, they specify importance. by default, they are sized differently, but you can modify that through CSS. H1 is the most important, H6 is the least, etc. In general, you should focus on h1 and h2's mostly. make sure they are descriptive and include desired keywords if possible. I've heard that they may not effect SERP's specifically, but probably ties into the algorithm that if keywords in a paragraph are related to the heading, they could get more weight, etc.