NOTE: From the wording of the original question (posted below), this isn’t a question about the meta description (which should be up to 120 characters in length).
No, instead, this is a question about the in-content, on-page product description that the user can read.
At the ~31:51 mark of Google’s August 27th English SEO office-hours, John Mueller, who’s a search advocate at Google, read a question that someone had submitted:
“What would be an ideal product description length for e-commerce sites offering quite technical products? Does the length of the product description influence how these pages rank at all?”
John’s answer was:
“I don’t know…you won’t like the answer, but…it depends.”
“You Can Use Whatever You Want…”
Mueller also said, “It’s something where, essentially, you can use whatever you want as a description, and some sites don’t even provide any description at all…they essentially leave it completely to Google or search engines to figure out what the description should be, but it’s something that’s that’s very unique to each individual site, and the length that they supply there that can vary…”
So…What Is It?
I know this may sound confusing. Should you even bother with the product description?
My answer, unequivocally, is yes.
It just makes sense: for a deeply technical product, the visitor is going to want to know as much information as possible: things like size, weight, speed, SKU…whatever pertinent data applies to your product, its use, and intended market should be made unmistakenly available.
Make your product description as long as it needs to be. I think that the longer it is, the better.
I think that because of what Mueller continued to say: “It’s also the case that in the search results, the lengths that we show varies quite a bit. Sometimes, we show something very short…sometimes maybe two lines…sometimes maybe even four lines or longer in the snippet in the search results, so it’s not the case that there’s this one number that you need to target.”
Product Description Length and Ranking
About ranking, Mueller went on to say, “And regarding ranking, it doesn’t change ranking at all, so you can supply whatever you want there.”
While the description length may not affect ranking, remember that, when they’re on the search results page, the searcher will be shown some text, and they’ll want to see something that will entice them to click to your site.
That’s why what John says next is also pertinent:
“The other thing maybe to keep in mind here is just the snippet that we show is not just the description in the meta tag. It can also be something from the page, and it can vary depending on the query…”
So, there’s no fixed or recommended character/length for the product description. That said, you should of course have one. Make it as extensive and comprehensive as possible–heck, maybe even have your user manual in there! The more extensive, the more text Google can pull to show to your prospects.