What is the length of meta tag and role of meta tag in search engine?

155 characters is the length of mata tag for serach engine.

There are two types of meta tags
1) Meta description tag : limit for it is 160 characters.
2) Meta Keyword tag : limit for this is 200 characters.

Meta tags give search engines more information about a web page. This is implicit information, meaning that it’s not visible to visitors of the web page itself. These tags are also important from ranking point of view.

Title tags should be 60 to 80 characters and Description should be 150 characters and keywrods maximam 200 words

Nowadays Google only understands title tag of 70 to 80 characters & meta description tag of 140 characters.It doest not take keywords meta tag.But you should put keyword meta tag of 180 characters for Bing or other search engines

TBH Meta tags are a bit dated and pointless, I'd suggest investing time in social interactivity.

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