Facebook's best practices
for og:description only indicates text that is at least 2 sentences long and is generally between 2 and 4 sentences. Their summary of the field length for both og:description and og:title is vague. This is probably intentional, because Facebook may choose different text lengths to show in different views (news feed, posts, in comments, timeline etc).
I've seen recommendations around the web to use around og:description 55 characters optimally, with a max of about 200. For og:title, recommendations I've seen are from 40-55.
It's not exact, but I hope this helps.
-Melissa
Max number of characters for Open Graph tags
