To put the importance of your Site Title in perspective: It is the most prominent thing searchers will see if you are included in their search results! Google is well aware of this and highly regards your Site Title in determining whether your site should be included for a given search. Research over the years has revealed that the longest site title that Google will display and use in search matching is some 64 characters. However, this is not a bottom line rcommendation!

Years ago, while at an SEO Seminar on this very topic, I conducted extensive research on Google and found a little known article that demonstrated conclusively, that Google would display a site title up to a certain number of pixels! The results of the search below, that I conducted on the afternoon of 2/5/2020, helps demonstrate this fact.

Among the Google search results, screenshot below, for the search, “real estate washington state” (without the quotes) are items X, Y and Z. Note item X immediately below displays 64 characters, (the approximate maximum number of characters). Interestingly item Y immediately below it appears longer but has 63 characters. And very curiously item Z displays some 61 characters with elipses. This is due primarily to having more capital letters, which take up more space. This gives us insight into the pixels/space on the page which is what Google actually uses to limit the length of the title that will be indexed and displayed.

For practical purposes; it is relatively safe to limit your site title to 60 characters!

As a guide; Your site title should include your primary key phrases, messages and brand. With the most important items first. And since people will be reading it; ensure it is a coherent "headline"

You may be able to use as many as 64 characters, but you will have to wait and see your site in a google search to determine if you have avoided having your desired Site Title truncated with ... elipses in Google's search results. 

X: Northwest Multiple Listing Service - Current properties for sale 
(64 characters)

Y: Tips for Navigating the Washington State Real Estate Excise Tax
(63 characters)

Z: Washington Lakefront Real Estate & Luxury Lakefront Homes...
(61 characters with Elipses; which means not all the title was used or displayed by Google)

     Screenshot of Search Results

And! Just when you thought you knew it all... Google may surprise you by rewriting your Site Title!

Like they did with ours! On our site you can see:

 

Yet, in a Google Search Result, this appears! Not that we mind... It is actually very good! :)