What happens if my meta description is too long?

1. Meta description explains what your post is about in SHORT. A user usually has a short span of attention, hence might skip the blog by having to read a long intro (meta description). So, description needs to be short, simple and catchy/ convey the essence of the post in the shortest way possible

2. There are search engine bots running always in the backend to group various content in the cloud. If a meta description is optimized, with keywords and is short and simple: these bots find it easily and your posts have better chances of showing up in search results.
if it is too long, chances are these bots may miss out on reading it fully or oversee the keywords and move to another content. This, you may miss out an opportunity to get the potential visitor to your post.

Try to write your meta description between 70 - 155 characters, although google doesn't say that it is a ranking factor but it will surely help you increase your Click Through Rate, which will eventually increase your traffic and your rankings will go up, just make sure that you do mention the keyword in your meta description as well and your description should be a summary of what your page is about!