If you want to control which pages get crawled and indexed by Google see my article for beginners to the robots.txt file.
H1-H6: Page Headings
I can’t find any definitive proof online that says you need to use Heading Tags (H1, H2, H3, H4, H5, H6) or that they improve rankings in Google, and I have seen pages do well in Google without them – but I do use them, especially the H1 tag on the page.
For me, it’s another piece of a ‘perfect’ page, in the traditional sense, and I try to build a site for Google and humans.
<h1>This is a page title</h1>
I still generally only use one <h1> heading tag in my keyword targeted pages – I believe this is the way the W3C intended it to be used in HTML4 – and I ensure they are at the top of a page above relevant page text and written with my main keywords or related keyword phrases incorporated.
I have never experienced any problems using CSS to control the appearance of the heading tags making them larger or smaller.
You can use multiple H1s in HTML5, but most sites I find I work on still use HTML4.
I use as many H2 – H6 as is necessary depending on the size of the page, but I use H1, H2 & H3. You can see here how to use header tags properly (basically, just be consistent, whatever you do, to give your users the best user experience).
How many words in the H1 Tag? As many as I think is sensible – as short and snappy as possible usually.
I also discovered Google will use your Header tags as page titles at some level if your title element is malformed.
As always be sure to make your heading tags highly relevant to the content on that page and not too spammy, either.