155 to 160 is the accepted norm. The 155 number is what Baidu cuts you off at, google is 160... Bing and Yandex allow more but really who cares.
When different browsers accept different limits on meta sizes, I go with whatever is smallest unless it's ridiculously absurd.
For example Baidu only allows for 64 characters of keywords meta, but ignores words past that not the whole meta, so there's nothing wrong with the 96 every other browsers seem to agree on... and with Google ignoring anything more than 8 words and 96 characters (leading everyone to THINK that the keywords meta is ignored entirely because nobody uses it right) I just go with 7-8 words/proper names and 96 characters as the limit for "keywords". (remembering of course relevance to content as well)
But yeah, for description, one to three natural language sentences of no more than 155 characters is good. And don't go stuffing it with keywords as that can get you pimp slapped for abuse. I see so many know-nothings who slop the same content into both "keywords" and "description" and it's like "NO F***WITS, THAT'S NOT HOW THIS WORKS... this isn't how any of this works!"