Snippet gives an idea about the content of the page. Viewers can decide if they want to read the whole article or not on the basis of the snippet. However many SEO company have observed that a 155 character description is not enough to give an idea about the content of the page to the viewer, this is specially observed in case of long tail keywords. Hence the size should be increased.

Even Google mentioned this on their blog post of 24th March: “When you enter a longer query, with more than three words, regular-length snippets may not give you enough information and context. In these situations, we now increase the number of lines in the snippet to provide more information and show more of the words you typed in the context of the page.”

It is delighting to know that even google agrees with the SEO content writing service providers. But then we are posed with the question What should be the Perfect Length of a Snippet? I haven’t experimented using longer description tags but think we can do so. However your decision to write longer descriptions or not should be based on your observation. If you find Google showing up longer descriptions on the SERP then I think you can go for it.
 
Nevertheless one important facet of using a longer tag means visitor will stay longer on Google and may not find the need to read the article or the page anymore. The description can be written using inviting language with a few keywords and should be partially inviting.