Nowadays Google states that meta title and meta description length does not depend anymore on the number of characters only, but even on the width in pixels.
The max meta title length is up to 570 pixels on desktop and 550 pixels on mobile, which might allow for up to 70 characters.
The max meta description length is up to 930 pixels on desktop and 905 pixels on mobile, which might allow for up to 160 characters.
Using the Metainfo Dashboard of JSitemap you can't go wrong at this, indeed it has an extremely powerful system that instantly calculates the pixel width of values entered both for mobile and desktop SERPs.
If you want to write great meta tags, here you find precious recommendations:
Meta descriptions need to be targeting searched keywords. Otherwise, Google is more likely to generate the description from site content. You still can keep the meta description clear, simple and eye-catchy so it will stand out in between other extensive result snippets.
Try to put all of the most interesting information in the first 120 characters so the information will be displayed in any other search engine, mobile devices and social media. Remember, the goal of your meta description is to attract the visitor to click through to your site.
You should stop writing longer meta titles and descriptions. Keep in mind that Google will cut extra characters using '...', which is why explaining a concept beyond this limit could prevent a visitor from understanding the meaning of the whole sentence.
How should i measure the correct size of meta title and description?