Google SERP Preview

On desktop, a title is cut at roughly 600 pixels and a description at roughly 990 pixels, both on a result that sits in a fixed column. Mobile gives more room rather than less, even though the screen is smaller, because Google wraps a mobile title onto a second line and runs the description over more lines: roughly 920 pixels for the title and 1,300 for the description. Desktop is therefore the tighter of the two, and a snippet that fits there fits everywhere, which is why the meters read against it.

Because Google truncates by rendered width, and characters are not the same width. "WILL THIS FIT" takes far more room than "will this fit" at the same character count. A character limit is a rough proxy; the pixel measurement above is the thing itself.

Often, but not always. Google rewrites titles when it judges another heading on the page fits the query better, and it rewrites descriptions more frequently still. Writing a title that plainly matches the query, and a description that answers it, is what makes a rewrite least likely.

Google bolds words in the snippet that match the searcher's query. Enter a target keyword above and the preview bolds it the same way, which shows how much of your description will visually connect to the search. That bolding is a real part of why a matching description earns more clicks.

Yes. Enter the URL and the preview fills itself from the live page, so you may see how it renders today before deciding what to change. For a whole site at once, the meta tag checker takes a list of URLs and scores them together.